Chara.

In this timeline, Chara is very much alive. Frisk falls into the underground not too long after chara. Azriel is still alive so there's no flowey. Everyone was packing up their things to go to the surface. Chara had already broken the barrier. Frisk fell and was out cold for awhile. Chara goes to see what was going on and Finds Frisk lying there. You are Frisk and Chara just woke you up. You decide what happens next.