@Little Brown Fox
Off the top of my head, there's Tenma, Hoshie, Uran, Reno, Atom's old schoolmates, several OCs, Ochanomizu (although he passed away during Atom's college years), and probably several old villain characters (but they're not really the focus here).
Hoshie is alive and divorced to Tenma, but she has a weird distant relationship with Atom. She never really learned to cope with his resemblance to Tobio despite being a different person, and sees him as painful reminder of her son's death and ex-husband's terrible behavior. Atom also has most of Tobio's memories in this AU, thanks to Tenma running off with Tobio's dead body to the Ministry and copying some of his son's brain into digital data before the cells degraded too much to scan. Fun stuff! It's a little more plausible than getting human memories from DNA, though (glares at the 2009 movie). Atom has had difficulty coping with these glitchy human memories in the past, but currently rationalizes them as Tobio's memories and not his own. I'm also not sure how close Atom is with his elementary school mates anymore, as it has been Quite A While...
On a lighter note, I have more comics and scribbles to dump oops
Photoshop crashed right after drawing these so unfortunately I can't fix some of the weird lines and coloring