Undyne

Undyne is one of the main characters in Undertale.

The player first hears about Undyne during their second encounter with Papyrus, where the player comes in just as Papyrus is about to say something about her but then notices you. Undyne will make an actual appearance when the player enters the Waterfall area past the first waterfall. When the player enters the patch of tall grass, Undyne will be seen on the ledge above the path. If the player spared Papyrus earlier on, he will appear to report to Undyne that he fought the human but failed to capture them. When Undyne states that she'll take the human soul herself, Papyrus will attempt to discourage her only to be promptly commanded to carryout his orders. Undyne will then appear throughout various points in the Waterfall area to attack the player, with one of the Monster kids from Snowdin following you along in the hopes of seeing Undyne fight. When you reach the end of the Waterfall area, the player will have a final confrontation with Undyne that will play out differently depending on the route the player has taken.

If the player is taking the Genocide route, the Monster Kid will confront the player about their actions and proceed to stand in their way. When the player attacks, Undyne will appear at the last moment and take the blow, saving the kid. But just when it seems like Undyne will turn to dust, she will slowly start to regain her form saying that she feels a burning feeling within her soul that won't let her die. Realizing that the threat is far greater than she thought, Undyne will feel the hearts of Monsters and Humans alike beating as one with the common goal of stopping the threat that the player has become. She will then transform into Undyne the Undying, the heroine of the Genocide route's story, and fight the player. When the player finally defeats her, Undyne will be surprised that even that power wasn't enough to win but will not lose hope. With her dying breath, Undyne will tell the player that Alphys was watching the fight and that by now will have begun evacuating the Monsters to safety, and that once Asgore absorbs the six human souls he'll have the power to ensure that the world will live on.

If the player is taking the Neutral route and they have killed Papyrus, they will meet Undyne at the entrance leading to Hotland. Just as she's about to recite the history of the Monster Kingdom, but will stop herself and instead point out that Papyrus didn't come to his meeting that day nor has he been answering his phone. She will demand to know what the player has done to Papyrus, and make it clear that if they try to proceed, she will kill them. When it seems that Undyne is defeated, her determination will keep her alive a little longer and will trying even harder to fight back. But as the player continues to attack, Undyne's body will falter more and her attacks will weaken even as she remains determined to stop the player. Before she completely turns to dust, Undyne's body will reform momentarily and will give a final cry of "I WON'T..."