IDK if this would be up your alley at all but I'm vaguely pondering throwing a 500-year-old fox trapped in a teenager's body at him because Basically Ageless Supernatural Shenanigans. I'm only vaguely aware of XXXHolic I think I watched a few episodes however many years ago that was??? so I'd need to collaborate to figure out an AUing-together scheme, loose trajectory, and how exactly to start, but I'm sort of imagining her showing up incognito as her Hello Fellow Teenagers identity to investigate him after hearing rumors, and ends up taking a liking to him? I'm spitballing, but let me know if this sounds like a thing that makes sense and/or could be interesting to pursue!