Board Thread:General Discussion/@comment-27907368-20160412010659/@comment-5815618-20160424164432

Chara is revived when Frisk falls into the underground. Chara is simply "guided" by Frisk. This, along with the fact that Chara's stats are the ones you see, implies that the SOUL you control is Chara's. Furthermore, after destroying the world the first time, Chara talks directlt to YOU, the player, not Frisk, and asks for YOUR SOUL, which is in fact, Frisk's SOUL (since you have Frisk's SOUL, you have the power to control Frisk).