Board Thread:General Discussion/@comment-27907368-20160323193924/@comment-26887879-20160510223427

When a human dies, their Soul leaves their body and settles in new human that is yet to be born. If human dies underground, the Barrier prevents Soul to do that. When Asriel was being beaten up buy humans, Chara's Soul left him and moved to a newborn human (Asriel could go back underground because anything can enter trough Barrier, but nothing can escape it). Eventually, after several births and deaths, that same Soul settled in Frisk.

Save/Load/Reset ability is possible only underground.