Board Thread:General Discussion/@comment-4504826-20160516034744/@comment-27907368-20160516043811

The Asriel and Chara being is mostly SOUL-bound - recall that Asriel absorbs Chara's SOUL. Without a physical vessel, the save/load/reset (SLR) ability does not occur.

There is no indication that a monster was ever able to SLR, even in the time before humans had fallen. When Flowey talks about determination, he is specifically talking about determination within a more physical form.