Board Thread:Questions and Answers/@comment-37807741-20190111181155/@comment-38040690-20190113003237

I believe Frisk left the underground with Asgore's SOUL. Frisk probably absorbed it when Flowey killed Asgore, right before Photoshop Flowey fight. That is my best explanation for neutral route. In genocide route, Frisk never leaves the underground, as she is presumably killed by Chara.