Board Thread:General Discussion/@comment-31067473-20170930204156/@comment-28517148-20171001181416

Ancient Glyphs say that anything can enter through the seal, but only beings with a powerful SOUL can leave, so human with enough strong Soul can pass the Barrier, like Frisk in neutral ending. Also, that being actually can has weak Soul or be even soulless and still be able to cross the Barrier, but then it must be in possession of borrowed Soul, like Asriel who took Chara's Soul and went to surface.