Board Thread:General Discussion/@comment-28925112-20160730202210/@comment-4560646-20161211033238

Snailmailfromtoriel wrote: @OpelSpeedster

1) Toriel might have left Asgore after the first human was killed.

2) Only determination souls can come back from the dead. Red soul isn't determination. Determination is a substance, not a soul trait. The Red soul being Determination is purely fanon.

Alphys says determination is an essence found in all human souls(she even extracted determination from the captured souls, none of which are red!). All humans were able to save, as hinted by Toriel if you tell her your pie preference and then reset or load to a point before she asks you that - as well as why Asgore doesn't get confused when you tell him he killed you X times before.