Board Thread:Questions and Answers/@comment-27372343-20160225202002/@comment-26347196-20160225203058

By default, we control Frisk. At the end of True Pacifist route, Frisk's name was revealed. Chara gains Frisk's control in the end of the Post Genocide Pacifist route.

I guess you can say in the Post Genocide route, Chara was technically in control, but they let you control Frisk anyway until the right time comes.