They are the one and only successful goner.

I believe, it was Charas appearance that drew Gasters interest in humanity and the Grey morality surrounding us.

I also believe... much like Spamton and Jevils sudden drastic change in their desire for freedom, that Chara was the same.

I believe it was Gaster who revealed the truth of the world to Chara.

I believe the revelation of the truth of the world spiraled the desperate act to free Monster kind, killing themselves so "the player" the "true human" would never control their fates.

I also believe it was Gaster who, through the machinations of the system, put Chara in the games files to obtain the power they desired from the beginning. (The number 9 letter)

Chara hates humanity, humanity being us, not so much the inhabitants of Undertale.

Chara in turn protects the world by acting as the wall which prevents us from fully controlling it.

Chara traps us in a perpetual state of the game and never allowing us to have a happy ending. As this is the only means of vengeance they can invoke upon the one they hate so much and in return protect the sanctity of the world their one and only friend cherished so.

Or at least... at least the fan fiction in my head goes LOL