Congratulations. You are Chara.

When you, Asriel and Frisk broke the barrier and freed all the monsters, you immediately punched Frisk for reset timelines for curiosity and then started killing everyone at all, and they blamed you for it. Then you treated him rudely, which made him feel like shit.

Soon, your Determination was also restored, and it turned out to be more than Frisk's. So you beat him even harder, but this time for insulting puns.

Time soon passed, and the incident and betrayal from Betty (Bête Noire) happened. You and Frisk instantly became as hostile as possible. You were barely kept from a direct fight.

It just so happened that Frisk went for a walk at night, and because of this (he did it without asking) you were forced to keep an eye on him. And because of the rain and his bad mood, he got sick. You still care about him, so you're going to help.