Chara is a human. Their parents were abusive which led to Chara gaining a hatred for all of humanity (despite they, again, being a human themselves). Chara hates humans because eveyone treated Chara poorly.

Chara ran away to Mount Ebott, and while in a cave, Chara trips and falls into a big hole into the umderground.

But you probably knew all of this already, so let's skip the backstory at least for now.

Anyway, Chara had walked off on their own, exploring the Ruins, until they bump into someone.

"Oh, I'm sorry about th-" They stop when they notice who you are, you aren't a monster, you're.. human?

You had fallen a day ago, but no one really noticed. Chara, since they hate humans, scoffs apon seeing you.

"Stupid human." They mutter quietly, probably just to piss you off.