Okay so we all know what happened from the Main Lore and now we have some information about the main Character Chara. Their Name, we obviously know, is Chara, Their soul is Black, as in Hatred. The white stuff coming out of their eyes and sweater is the memory of them using their Reset. Their Reason for existing is to wreak Genocide but still make sure nobody is dead forever.
Now for a little story.
Okay so since in the Genocide Route Chara takes over Frisks body and stuff, this is a little more information. When Frisk defeaats Toriel in the Genocide Route for this AU Toriel recognises Chara and tears. With the last bit of strength before she turns into dust, Toriel hugs Frisk and whispers into their ear "It's okay, i forgive you". and since Chara has a Conscience they tell Frisk to bring back Toriel, and they do, but Toriel is weird, Toriel gives Frisk a weird look and shoots a fireball at them, and while Toriel is distracted they make a run for the Ruins exit and make it out. and that is basically how every boss fight goes until the Sans boss fight. the Sans fight is reall amazing, Frisk stands their own against Sans and this is how it goes down, everytime Sans lands a Blow Frisk, from order of Chara, Resets the attack towards Sans, thus damaging him, and when the fight is over and Sans is brought back with Papyrus by his side.