Undertale spoilers
@kara_dreamer@witches.town Ah right, well... in which case it follows on from all that. Basically Asriel has no soul after he dies, joined with Chara. The two are buried in the flower patch. Due to their joining, Asriel's soul persists on, as does Chara's, what with both being so full of Determination now.
In her experiments, Alphys injects "a random yellow flower" from this patch with Determination, to see what happens if a thing with "no soul" is given Determination.