Found By Jasper by Kaje Harper

Jasper’s curiosity has carried him through adventures with ghouls, demons, world gates, and megalomaniac sorcerers. He plans to spend the next few years cataloging the mansion’s vast and moldering library, and experimenting with magical construction. Professor Xsing, the raccoon not-quite-familiar who’s decided to work with him, has fascinating opinions. Hamish, the sorcerer they rescued, is becoming a friend. It’s silly for Jasper to wish for anything more.
Hamish spent months chained in one small room. Now he lives free in a mansion full of books and friendly people, but adjusting isn’t as simple as it should be. When his past weighs heavily, he can ramble the woods and fields. When he can’t sleep, there’s a nocturnal raccoon happy to share his explorations. But he’s lonely, he misses his own long-lost familiar, and when he thinks he hears her ghost— even though familiars don’t have ghosts— his life is about to be overturned.
Something’s afoot— ghost, spirit, familiar, wood mouse? With Silas, Darien, and their familiars headed off to France, Jasper and Hamish join forces to help those in need. And as they work together, an unexpected chance at love may fill the lonely corners of both their hearts.

Found by Jasper is a novel in the Necromancer series, with our past heroes as side characters. Older MCs, friends to lovers, first times, demisexual.
**Content warning for secondary character self-harm.**