Undertale Mix Lore?!?
Everything takes place in a universe where in a genocide route Frisk does not find Chara.
Frisk still has that feeling to continue killing, so Frisk returns to the beginning of the Ruins.
Looking for monsters, finds a fallen human, a human with blue hair.
In this universe, a deceased human can take control of a living soul with enough determination,
So Chara with Frisk's determination, takes over their body.