Board Thread:General Discussion/@comment-26456251-20160824191731/@comment-27089028-20160828182013

we play as Frisk throughout but at the end of the Genocide Route Frisk loses control of their body which is now controlled by Chara who after you gained level 20 was strong enough to catch Sans by surprise and end him and then after walking in on Asgore and Flowey completely took over.

In a Soulless Pacifist route, Chara gives control of the timeline back to Frisk allowing them to reset at the price of their soul. After you achieve the True Pacifist ending Chara can instantaneously take back full control.