AOT Characters Playing Undertale
Eren: Would (as soon as he sees Flowey) dislike all of the characters automatically and do a full on genocide run. Maybe will do a pacifist run if curiosity takes over his actions.
Fav Character: Chara, Frisk or Undyne
Mikasa: Would follow Eren, but deep down inside would want to do a pacifist run.
Fav Character: Toriel or Undyne
Armin: Pacifist. It would take Eren or Mikasa's persuading to make him do a genocide run.
Fav Character: Alphys
Reiner: Pacifist, but would do genocide to show off.
Fav Character: Asgore or Papyrus
Bertholt: Pacifist. Would have a hard time trying to do a genocide run afterwards though.
Fav Character: Toriel or Asriel
Annie: Would play genocide, but in her spare time, would play pacifist and actually enjoy it.
Fave Character: Undyne
Jean: Would automatically do a genocide run, but once he begins a pacifist run, he will like it way more the genocide, but won't admit it.
Fav Character: Sans or MAYBE Mettaton
Marco: Pacifist. Wouldn't dare to even do the genocide run.
Fave Character: Toriel or Asriel
Connie: Genocide. In the middle of it though, he would regret and want to do a pacifist run.
Fav Character: Papyrus or Undyne
Sasha: Most likely pacifist. Would figure out the little secrets hidden in the game. Won't mind doing a genocide run, but will be reluctant.
Fav Character: Lesser Dog or Temmie :3
Krysta: PACIFIST. Would NEVER do genocide.
Fav Character: Toriel, Frisk, or Asriel
Ymir: Pacifist along with Krysta (of course). If asked to do a genocide run, she would do it in her own spare time.
Fav Character: Undyne
Erwin: Pacifist, but would need convincing to play genocide.
Fav Character: Asgore
Levi: Would do a pacifist run if he wants to, would do a genocide run if he has to.
Fav Character: Sans or Washua ;)
Hange: Would exploit the game as well as find all of the hidden secrets. Do both genocide and pacifist for the fun of it and get the hacker ending. Eventually find a way to do a complete memory wipe of the game.
Fave Character: Frisk, Chara, Flowey, or Alphys
Petra: Pacifist. Would be pretty unhappy about doing a genocide run and will defend her opinion about it.
Fav Character: Frisk, Asriel, Toriel, or Papyrus