Board Thread:General Discussion/@comment-27907368-20160506131212/@comment-27620479-20160513205547

If Frisk's essence is "determination", they cannot be true to it on a neutral run. No ending that is possible on a neutral run does really change fate. Monsters remain trapped, and Frisk doesn't get to go home either, unless the barrier only measures soul power and they have enough extra determination.

A pacifist or a genocide route is more true to the essence of determination. We know Frisk's soul refuses when fighting Asriel. It should also be in pretty good shape when Chara takes it after a genocide run. Most likely, after Chara either attacks Frisk and deals much more damage than what they can take, or destroys the world they are in. Also note, that before that, even an attack that is just strong enough to kill Frisk, already shatters their soul.