Undertale - The child has eaten the bagel. He will never belong to us now.

Sure, no problem.

"Chara" is the name that people have given to the "first child" – aka the kid that Asgore and Toriel adopted and whose soul Asriel absorbed. They are also the person that you name at the start of a new game, so what their name will be in a given playthrough will vary. The "Chara" designation comes from some of the game's promotional material, and the fact that if you name the kid "Chara" in your game you will get a message designating it as "the true name."

Frisk is the player character, the one you have control of throughout the game.

The designation gets confused because of what happens in certain playthroughs. Chara is indeed the sociopathic one – as Asriel says if you visit him at the start zone after a pacifist run, Chara "hated humanity" for reasons we aren't told. They do, however, look a lot like Frisk, and many of the characters mistake one for the other because of that. Also, in a genocide run Chara's disembodied soul invades Frisk's body and eventually takes over, which blurs the distinction even more.

That being said, it's suggested that the pacifist playthrough is the "true" run, since that's the one where you get Frisk's name – implying that pacifist Frisk is who they "really" are.

I hope that helps.