Board Thread:Questions and Answers/@comment-37807741-20190111181155/@comment-26780009-20190129202944

It's simple. Asgore claims that they need seven human souls to destroy the barrier. There were six humans souls before Frisk, there were still six humans souls after Photoshop Flowey. Frisk's also got a human soul. That makes them seven, enough for Frisk to cross or destroy the barrier freely.