Board Thread:Wiki Discussion/@comment-26843781-20160301110343/@comment-26887879-20160519223757

Well, I think that Undyne's and Asriel's eyes obviously have to do something with them being undying/determined. But that doesn't necessarily means Sans fits their category, we can't know for sure... Especially since weaker monsters like Froggits and Napstablook have those eyes...

I don't think Sans is soulles. Flowey/Asriel absorbs his soul during TP run and he appears as one of the "Lost Souls" during that fight...