I've been having super brainworms, so I filled out @chipper-smol's Loop template! The story of my Loop doesn't touch on a lot of these questions, so it was fun coming up with answers! (Alt text explains slightly more!)
I've also made a billion sprite edits, so more of those + lore under the cut! (Self harm/suicide TW, 2hats spoilers)
So! I am working on a fanfiction (which I hopefully finish at some point) that is mostly a post-canon headcanons thing, but also has Loop's whole thing going on!
Basically, after Two Hats, Loop makes a wish that they're able to keep their promise to see Siffrin and his friends again, and ends up in his old body. Which is Siffrin's body. And that's very dysphoric and re-traumatizing on the whole, feeling like someone else is living their happy ending thing.
Loop is still using the name "Loop" because, as much as the loops aren't a happy memory for them, helping Siffrin (and eventually, the rest of the party) through the House as "Loop" is the only part of this timeline that belongs to them - the only thing they can be recognized for, their place in this story. Everything before then - that belongs to "Siffrin" now, they can't own that anymore, none of the memories others have of those times belong to them now, they belong to the other Siffrin. (And, they can't even remember those times well, though leaving Dormont starts to bring back some old memories of that journey...)
Loop, still looking too much like Siffrin and unsure how to conceptualize who they are in this world where someone else took their place, avoids reuniting with the party for a while, but eventually bumps into a newly changed Isabeau who escorts them to the gang's 1 year reunion (they finally did split ways after their second journey). Over this mini-journey with Not-Isabeau-Anymore, Loop is generally... A lot. They have done less healing than Siffrin has at this point, and they have a hard time remaking the relationships they lost to the other Siffrin. The fact that Not-Isabeau wants to start over with everyone as his new self only makes it easier sometimes. (The fact that Loop has gone from the star-person Isabeau knew to a guy who looks suspiciously like his recent ex also doesn't make it easy.)
Trying his best to help Loop deal with Everything, Isabeau suggests Loop try changing - Loop doesn't want to throw away who they are (the Universe already did that for them), but body craft can help them reclaim what the Universe took from them: the choices they made, the signs that they hurt themselves, killed themselves, suffered and fought and struggled through all that time that doesn't exist anymore. So they craft scars onto their body - decapitation scars on their neck, lines down their wrists of hypothetical tally marks they would have kept if their body had persisted between loops. (This makes Isabeau very uncomfortable. Loop doesn't care about making him uncomfortable.) (They didn't take their dagger with them, by the way. It's too easy to consider using it, when they think about how the future they toiled so long for doesn't belong to them anymore.)
So yeah, Isabeau comes to the reunion to re-introduce not only himself, but another old friend (who, he maybe asks Siffrin to take off his hands because Sif, Bud, Exie, I Don't Know How Much More I Can Take Of This Guy...) (Loop isn't making themselves easy for Siffrin to handle either - tee hee your ex-boyfriend made me crafted clothes~ hey if you fumbled him that means I can take the dream for myself can't I~? How's having everything I wanted? Stardust stop hugging me I'm being unpleasant. I-I'm not crying.) (Loop wants to become friends with everyone again, they just don't know how to deal with the Everything and is very abrasive about it. They'll get better. Eventually. We hope.)
