Board Thread:Questions and Answers/@comment-27327020-20160404174206/@comment-25181180-20160505022939

Chara is revived only after we've fulfilled the kill counter by delivering the finishing blow to Flowey; meaning that they won't exist without us. Chara is also tailing he player, sucking Frisk's ability in the end of the story and leaves our SOUL defenseless. With a 99-attack Knife, a huge determination (roughly equal to a Genocide Frisk), and a defenseless SOUL, they can simply destroy us with than 9 attacks.

Asriel has 8 souls, 7 from the previous humans and from the monsters' (as every monsters equals one human). And Chara is a part of it. If you were saying that Chara is stronger, then Asriel could simply destroy the Underworld and half of the surface in one hit; but he didn't.

It is not really confirmed that Chara erased the Underworld; they might've manipulated the player to believe the world is erased while they took Frisk's body (similar to how Chara tricked Asriel).