Board Thread:General Discussion/@comment-5413572-20160521065337/@comment-27907368-20160521084603

The ghosts only fuse with their forms in the Geno route - in both cases due to an extreme selfless emotion.

Sans and Papyrus are unique in that they're rhe only paired species of main characters. However, they're also unique because they're incredibly quirky and serve polarized ends of the spectrum. Though Sans certainly shows some unique traits in Geno, there's little to indicate that the skeleton race utilized determination to become... fleshy and human like?