Im curious to know your take on this head cannon. My idea as to why Fell!pap makes Edge ware a collar isn't just due to trying to deter other monsters in their Au from killing him, but also apart of Fell remembers each timeline when the human killed him, so in a way Fell is trying to keep Edge's Neck protected and make it harder for the human to decapitate Edge like the human did pap.
*Well, I think in the original Underfell, the spiked collar was more a fashion choice than a meaningful item, but when the AU was given over to the fandom, lots of headcanons abounded. Some people viewed it as a simple ownership thing between Underfell Sans and Papyrus. Some people (myself included) viewed it as a symbol of Sans being under Papyrus’ protection due to his 1 HP in a kill or be killed world.
*If you dig up some relatively ancient Underfell Sans art on @cheapbourbon‘s blog, they drew a really amazing piece to the effect of: “Spiked collars were placed on sheep dogs to protect their necks from wolves” with what appear to be Underfell Papyrus’ gloved hands being impaled upon the spikes. I actually really love the imagery especially when you read the original tags.
*As for this headcanon, I love the protective brother trope in Underfell. I love the idea that Underfell Papyrus has an awareness of his own fate during a No Mercy route since that is hinted at in the actual game, and that he would instinctively want to protect his brother even if he didn’t have specific memories of why.