I found your Swapfell MH character info and like, saw Asriel's and have a thought.
If his home is in New Home, imagine if he still appear some time after Waterfall, maybe he's trying to prank random people 24/7 but you're on the same way as him so more than half his prank landed at you, so in Hotland you'll get like, both him and Undyne trying to be villain, and for some reason I feel like he'll have more success rate then the royal scientist lol
ooh I love the idea of Swapfell Asriel actively causing mischief throughout Chara's journey! though I feel it'd be overcrowded if it continued into Hotland :'D
I can imagine near the end of Waterfall one of his pranks accidentally backfires on Alphys, who threatens to report back to Toriel if he doesn't leave. there could totally be room for him to pop up again in Hotland on his way home (either still being mischievous or just sulking), but I'd rather have that be Undyne and Napstabot's time to shine.
it's an excellent thought though, thank you ✨
(also this is a good opportunity to mention that I haven't forgotten about Swapfell MH and I do plan on getting back to it! I just can't make time for it for a few months at least 💔)