Toriel

Toriel is one of the main characters in Undertale who, arguable, has made some of the biggest impacts on the events of the game. While Toriel does understand most of the game's mechanics, she doesn't have any direct control over them as do some of the other characters nor does she seem to recognize them as "game mechanics."

At the start of the game, Toriel will be the second character the player meet as she will save them from Flowey. Saying that she passes by the area everyday to see if anyone has fallen in, Toriel will guide you through the Ruins and give a proper explanation of the game's mechanics such as the battle system, storage and even gives you a cell phone. At first, Toriel will hold the player's hand through most of the Ruins' puzzles and traps, but then she will leave them alone and ask to wait while she prepares something. It is roughly at this point that the player can choose to go down either the Pacifist route or the Genocide route, with Toriel occasionally calling on the cellphone. When the player makes it to Toriel's house on their own, they'll arrive in time to see Toriel try to make another call only to be surprised to see you already home. While noting that you seem unharmed, Toriel decides that leaving a child alone in general wasn't a wise choice and shows you inside to reveal the surprise she's prepared; a butterscotch cinnamon pie and your own bedroom. When the player goes to speak with Toriel, she talk about how she'll prepare for your new life here until the player asks about going home. She'll initially brush it off by saying "this is home", but when Toriel realizes that the player is not going to be discouraged she'll excuse herself for something. Following after Toriel down the stairs and speaking to her will reveal that she's going to destroy the doorway leading out of the Ruins to ensure that you'll never leave, promptly asking you to return upstairs to wait. Continuing to follow Toriel will cause her to elaborate that other humans have left in the hopes of returning to the surface, only to be killed by Asgore. Following Toriel all the way to the door will result in her starting a fight to see if you have the strength to survive, where the player will choose to either spare her or kill her.

Killing Toriel on the Genocide route will result in her asking if you really hate her that much, followed up with the statement that she was actually protecting the Underground by trying to keep you in the Ruins. The player will then watch as Toriel turns to dust and her Soul breaks right in front of them. For the rest of the game, the player will only hear about Toriel on rare occasions from only a few other characters. The player's first encounter with Sans will be among them, wherein he'll say "gee, lady, you really know how to pick 'em" in response to Chara's lack of emotion at his prank. The next time the player will hear mention of Toriel will be at New Home, when Flowey explains what happened after they first awoke as a flower. According to Flowey, after not feeling any emotion after spending time with Asgore, Flowey found his way to the Ruins where he found Toriel. Flowey hoped that "she of all people" could make him feel emotion again, but that didn't come to pass contributing to his current world view. Finally, the player will hear mention of Toriel from Sans again in the hallway, wherein seeing that he's going to have to fight Sans will make the remark of "Sorry, old lady. This is why I never make promises".

If you choose to spare Toriel in the Pacifist route, she'll be confused and frustrated at the fact that your neither fighting back or running away.