Flowey
Flowey is a major character in Undertale, and a major antagonist depending on which route the player takes. Flowey is a self-aware entity, fully aware of the fact that he is a "character" in a "game" and even has some control over the game's mechanics.
At the start of the game, Flowey will be the very first character you'll meet. Introducing himself, Flowey will note how you're new to the Underground and that you're most likely very confused about what's going on. He will say that he will teach you how things work in the Underground and show the Battle mechanic, without saying that it's a battle. As Flowey's real intent is to kill you, he will respond with sadistic laughter should you get hit with his bullets or annoyance should you avoid them. Soon reducing the size of the box and surrounding you with a ring of bullets, Flowey will state his mindset of "Kill or Be Killed" before closing the ring on you, only for the bullets to suddenly disappear and Flowey getting knocked away by Toriel's attack. Flowey will be encountered again right after the boss fight with Toriel just before the exit of the Ruins. How the player's interaction with Flowey goes will depend upon which route the player chooses to take and how many times they've played the game beforehand.
In the Genocide route, Flowey will laugh a little at your actions and state that you're not really human, but "empty" like himself. Flowey will then ask if you are really "Chara", following up on that by saying that the two of you really are inseparable "even after all these years." Finally, Flowey will say that he has a plan to make you even more powerful than you are now with your "stolen Soul"; ending this world by killing every single Monster you come across. Before leaving, Flowey will bid you farewell by verbally saying "That's a wonderful idea" as opposed to more readable text.
The player won't have anymore direct interactions with Flowey until they arrive at New Home and start working their way to the castle. As the player looks for the keys to unlock the chain, Flowey will appear to talk to Chara. He will first speak of how the two of you use to play in this very house, and how today will be "just as fun." Flowey will then reminisce on when he first awoke into the world as a flower in the castle garden, scared at the fact that he couldn't feel his arms or legs, and called out for help that didn't come. Eventually, Asgore found Flowey crying in the garden and Flowey explained everything to him. But when Asgore started crying and telling Flowey that "everything will be alright," Flowey didn't feel anything having lost their compassion by becoming a Soul-less entity. Flowey then spent weeks with Asgore hoping that eventually he would feel something, but left when he didn't. Reaching the Ruins, Flowey found Toriel and hoped that "she of all people" would make him feel something again, but that didn't happen. Deciding that there was no point in living without feeling love, Flowey attempted to commit suicide but started feeling apprehensive just as he was about to die. Afraid of what will happen when one dies without a Soul, Flowey suddenly felt a strange power within and "woke up" back in the garden, at their "Save Point." After experimenting a little, Flowey found that as long as he "stayed determined" he could "restart" again from any save point. Initially, Flowey used this power to help out others but soon found that the people of this world could be very predictable, possibly even realizing that it's all a "video game" at this point. Soon, Flowey started killing others, at first to see what would happen if he did. While he found it "liberating" at first, Flowey soon grew tired of that as well but claimed that it was at least better than being someone who just stands by and watches it happen, possibly breaking the fourth wall by addressing people who watch video playthroughs of the Undertale saying "I bet someone like that is watching right now". By now, Flowey has apparently done everything the game has to offer him, but he has found Chara to be a refreshing experience; Flowey didn't recognize them when they came to in the Ruins, Flowey wasn't able to kill them right away, and Flowey hasn't even been able to load his "SAVE file" ever since the first encounter in the Ruins. After pondering how and why Chara "came back," Flowey will then state that his only wish now is to carryout the plan of killing everyone and ending this "pointless" world, but is now pondering the idea of carrying out their original plan of destroying the Barrier to go to the surface. Stating that Asgore would most likely give Chara the six human Souls he already gathered, Flowey will initially state how he's doing all this because Chara is still special to him. However, after stating that "creatures like us wouldn't hesitate to kill each other", Flowey will start shaking with fear remembering how he hasn't been able to reload his "SAVE File" and his fear of what would happen if he died as a Soul-less entity. That fear will be enhanced by Chara apparently advancing towards Flowey in a threatening manner. After Chara has beaten Sans and gets to the garden to confront Asgore, the king himself will be heard saying that he's never seen a plant cry before and say that Chara must be the one the "flower" was just warning him about. When Chara attacks, Flowey will deliver the final blow to Asgore in the hopes of being spared even showing his original face as Asriel. But even after all his tearful, and ironic, begging Flowey will be brutally killed by Chara without the player being given a choice.
If the player had killed Toriel and many of the monsters in the Ruins, but then reset their Save file so as to spare them all instead, Flowey will immediately make it clear that he knows full well what you did and mocks you for the fact that even your "power" will never be rid of the fact that you've killed someone.
If the player defeated Sans but then reset the game before completing the Genocide route, and then played through the Pacifist or Neutral route, Flowey will appear after Sans' phone call to ask why you "screwed it up" when the two of you were about to achieve "true victory." Wondering if it was some form of revenge against him, Flowey will then pass it off as you just looking to see what this world is like before destroying it. He will then say that he'll wait for you to finish "messing around" before carrying out the plan.
In the True Pacifist route, Flowey will mockingly call you clever for playing "by your own rules" and not kill anyone, but will follow up on it by asking what would you do should you meet a relentless killer. After dying so many times, would you kill out of frustration, or give-up on the game altogether? Flowey will say that giving up would result in him gaining the power to control the world, calling himself the "prince of this world's future" before leaving.
The player won't have anymore direct interactions with Flowey until the end, but they will have the opportunity to hear some more about him from Sans. When the player goes to leave Snowdin for the Waterfall area the first time, they can talk with Sans who will offer to treat you to a meal. Going to Grillby's to eat, Sans will talk about Papyrus at first but will soon drown out everything to ask you about a "talking flower." Explaining the Echo Flowers of the Waterfall area to the player, Sans will say that Papyrus has been interacting with a particular flower that seems to be doing more than just repeat what it just heard; a flower that's been giving Papyrus compliments, tips, and prophecies. Sans passes it off as someone, besides himself, playing an elaborate prank on his brother but this is most likely Flowey toying with Papyrus. Sans will then warn you to keep an eye out regardless before leaving.
When the player reaches New Home and explores the castle, monsters will appear to tell the story of how Asgore's policy came about which concerns Flowey's original identity. Long ago, Chara fell into the Underground and was found by Asriel, Toriel and Asgore's son. Asriel and Chara soon grew close to the point that they were like siblings, but then one day Chara died from illness. Asriel then absorbed Chara's Soul and passed through the Barrier, carrying Chara's body to fulfill his dying wish of seeing the golden flowers of his home village. Arriving at the village, Asriel tried to lay Chara's body on a bed of golden flowers but was spotted by the humans. Thinking that Asriel had killed Chara, the humans attack with everything they had but Asriel refused to fight back. When Asriel returned home, he succumbed to his wounds and died, prompting a furious Asgore to declare that all humans who fall into the Underground shall be killed.
Once the player has defeated Asgore, Flowey will appear to kill Asgore himself should the player spare him or thank the player if they chose to kill him. Either way, Flowey will then take the six human Souls Asgore had collected and absorb them, causing the game to crash. When the player goes to restart the game, they will immediately get a glimpse of Flowey's new power by seeing that the game's title has been changed to "Floweytale" and the intro cutscene has been changed as well. When the player goes to chose a SAVE File to start the game where they left off, the only File they will find will be titled "Flowey, My World." Opening the File, the player will find themself in a completely empty space save for one SAVE Point. Interacting with the SAVE Point will reveal the player's original SAVE File, only to watch it be destroyed by Flowey. Thanking you for having weakened Asgore, Flowey will state his intent to take your Soul as well so as to become the "God" of this world, clarifying that your old SAVE File is gone but he has made a "replacement" that'll allow you to see yourself die over and over again. Initiating a battle, Flowey will appear in a form consisting of photo-realistic components that contrast greatly with the rest of the game's pixelated art style, and will constantly throw attacks at you without waiting for a "Turn." As for the player, they will only be able to move about within a small portion of the screen with the "Fight" option sporadically appearing for a short time before disappearing again. Whenever the player dies, they will initially get the usual "Game Over" screen with Asgore saying that it's just a bad dream, only for Flowey to finish that statement with "...and you'll never wake up" and fill the screen with evil laughter before crashing the game again. Should the player last long enough against Flowey's attacks, the Souls will initiate their own attacks against the player but will give health boosts should you reach the "Act" option. Each time the player successfully calls out to the Souls, they will be able to deal more damage against Flowey until his Health Bar reaches zero. However, just when it seems that Flowey has been defeated, he will load a SAVE File where his Health is full and the player's health is at one, before delivering a "Final Blow" that's followed up by loading other Files of "Final Blows." Finally surrounding the player with bullets, Flowey will mock them for hopelessly confronting a god and dares them to call out for help when there's no one else to hear them. To Flowey's surprise, however, the six human Souls themselves will respond to your call and remove all of his powers before attacking him. Now heavily weakened, the player can choose to either kill Flowey or spare him. Killing Flowey will result in him smiling and saying "I knew you had it in you" before turning into a regular flower. Sparing Flowey will result in him warning you that he hasn't learned anything from his loss and that he'll destroy everything if you don't kill him, but will eventually run away from not understanding your kindness.
If the player spared Flowey he will appear before you after the phone call with Sans, wherein he'll question why you took the actions you did; even though you made some great friends, you ultimately set back their progress on leaving the Underground and you'll most likely never see them again. In a surprising act of kindness, Flowey will tell you that he may know of a way to get you a better ending by going back to your SAVE File from before Asgore's battle and leave to befriend Alphys, or Undyne if they player hadn't befriended her in the first playthrough. The player can initiate the path to the True Ending without having to spare Flowey, but this is the way they will readily know about it.
As the player traverses the True Laboratory, they will find some log entries that detail how Flowey came to be. In an attempt to overcome the limits of Soul power, Alphys sought to find a way to animate a Soulless entity to act as a vessel to house the human Souls. Choosing a golden flower in Asgore's garden as a candidate for the experiment, Alphys successfully animated the flower and was hoping to surprise Asgore with it, but the story Flowey told in the Genocide route indicates that things didn't turn out as she hoped. The player can also find some home videos of the Dreemurr family on VHS, including some regarding Asriel and Chara. Some show Asriel and Chara's sibling bond, while others indicate that Asriel helped Chara to deliberately become sick by eating some buttercup flowers as part of a plan to collect human Souls to set everyone free. Just as the player gets on the elevator to leave the Lab, they will get a call on their cellphone from an unfamiliar voice saying that everything is now in place and they will see you soon. The elevator will suddenly activate and take you to the Castle, the vines jamming the door indicating who's responsible.
Once all of your friends have gathered to stop Asgore from fighting you, Alphys will ask Papyrus how he knew to let everyone know to come to which he'll say "a little flower told me." Just as Alphys processes what he meant by that, Flowey will trap everyone in vines saying that he took all the human Souls while everyone was busy talking. Flowey will then mock you for befriending everyone, because that has allowed him to not only claim the human Souls but also have the chance to claim your friends' Souls as well. Flowey will then explain that the reason why he's still doing this is because if you "win the game" then you won't come to "play" anymore, so he's going to have get so close to "winning" only to take your victory away so that you'll keep playing. Trapping the player in a box so small that there's no space to move, Flowey will then proceed to attack you until your friends manage to use their powers to protect you. As the other monsters you've befriended throughout the Underground arrive to help, initially it'll seem as though Flowey was unprepared for this and is going to struggle. However, it turns out Flowey was hoping this would happen as it now gives him the chance to absorb the Soul of every monster in the Underground, turning him back into his original form of Asriel Dreemurr, the final boss of the True Pacifist route.
Despite his newfound powers, Asriel will actually remain consistent with game's "rules" while it is the player who breaks some of the rules, such as not "dying" when their health falls to zero. Throughout the fight, Asriel will state that he no longer wishes to destroy the world but will instead reset everything so that you will play the game all over again, mocking you for how you will actually do that because you "love your friends" and want to give them a "happy ending." Deciding that he's had enough "fun", Asriel will purge the timeline to kill you and start the game all over again, only for the player to survive with only one HP. While Asriel is impressed by this, he will warn you to not get cocky as he was only using a "fraction" of his full power. Changing into his "ultimate form", Asriel will finally break some of the game's rules by taking away your ability to choose different options on your turn for the fight. Asriel will also state that each time you die, your grip on the world loosens and your friends will lose some more of their memories of you. Just when it seems like there really isn't much that can be done about the situation, the player will be able to use their SAVE power to start saving their friends from Asriel's influence. Once the player has saved Toriel, Sans, Papyrus, Undyne, Alphys and Asgore, they will have the feeling that there is one more person to SAVE. This will result in both the player and Asriel remembering when Chara was found by Asriel, which in turn will cause Asriel to lose his composure and weaken his attacks. Failing to convince himself that he doesn't need anyone, Asriel states that the reason why he's trying so desperately to keep you around is because he doesn't want to go through the pain of losing someone precious again, unleashing an attack that leaves the player with only 00.0000000001 HP and "SAVE" as their only option. Calming down and turning back to his "child form", Asriel will realize that your not actually "Chara" as they died long ago. After learning the player's true identity as Frisk, Asriel will state how he hasn't felt anything like this for a long time. After having been an unfeeling flower for so long, Asriel can not only feel his own emotions again but also the emotions of all the monsters in the Underground, realizing just how much they all love Frisk despite not knowing them for long. Knowing full well the weight of what he's done, Asriel will say that it doesn't matter if Frisk forgives him or not as he'll have to give all the Souls back. But not before using the power of all the Souls to finally destroy the Barrier and set the monsters free. Saying that he has little time now that the Souls are gone, Asriel will tell Frisk that it would be best to just forget about him prompting the player to comfort Asriel or not. Just before disappearing, Asriel will ask Frisk to take good care of Asgore and Toriel for him.
Frisk will then awaken as Toriel and the others call out their name, revealing that Asriel has indeed restored the world to how it was with the exceptions that everyone knows Frisk's name, the Barrier is gone and nobody seems to recall Flowey's actions. Should the player choose to go all the way back to the Ruins before leaving the Underground, they can find Asriel at the flower bed where they first awoke at the start of the game and have the option to talk to him. At first, Asriel will simply say that it's for the better for him to stay there, but will soon elaborate on how and why things turned out the way they did. To start with, Chara came to the Underground because they hated humanity for reasons they never wished to speak about, and they carried that hate all throughout their life even as they were being given love by the monsters. Because of that, Asriel realizes that Chara was probably not that great of an influence on his life despite the fact that they got along so well. Since Frisk and Chara were so similar to each other, Asriel projected a little and tried to treat Frisk as though they were the same, when in truth Frisk would have been a better influence on his life. Finally, when Asriel absorbed Chara's Soul way back when, control over Asriel's body was split between the two of them; Chara was the one who picked up their own body to take to the village, and when the humans attacked Asriel stopped Chara from attacking. Dying from that then waking up as a flower combined with the experiences described in the Genocide route, as well as blaming himself for it all, contributed to his world view of "Kill or be Killed" as Flowey. But now, thanks to Frisk, Asriel doesn't regret his decision anymore considering that another war almost started but Frisk managed to convince everyone to change their minds about humans before being set free. Finally, Asriel will tell Frisk that they should head on back to the others, and warn that there are a lot of "Floweys" out in the world with whom "being nice" may not work out so well saying "Don't kill, and don't be killed." Asriel will appear one last time to start the "Special Thanks" list after the credits role.
After the "Special Thanks" have ended, Flowey will appear to say that you've finally achieved your happiness and that he'll keep his word of not taking it away. But, he will warn you that there is still one more threat to all this; you. You, the player, still have the power to completely reset everything back to the beginning as if nothing had happened and do whatever you wish, for better and for worse. That's the power Flowey wanted in the first place, but now, after everything that's happened, he no longer has the will to go for it again. As such, Flowey will ask you to just let everyone keep their happiness, but in the event that you do reset everything, you must erase his memories as well. Flowey will then apologize just in case you've heard this before, before saying "so long."