Harry
When Harry’s born, it’s all Regulus can do to stop not only himself but Sirius from being at the Potter Estate every day. He’s sure Lily and James need their parental bonding time and he’s sure that Remus could do with having Sirius actually at home for more than 5 minutes without bouncing up and suggesting that Lily and James need help with this or that.
Eventually, Regulus hears that Remus cracked and sent Lily a very harried (*author laughing at herself here sorry*) letter by owl that demands they set days of the week that Sirius is allowed to be at the Potter house without consequence (what consequences, Regulus isn’t sure he wants to know, but he’s quite sure it involves Sirius being made to sleep in his own bedroom without Remus and that’s from the complaints Sirius starts sending him not even two days after the agreement has been made).
Regulus keeps himself entertained by spending more time with Evan and Barty even though he knows he’d much prefer to be spending time with the tiny Potter baby who’s managed to make both Black brothers fall in love with him without even trying.
Evan and Barty have set up shop in Diagon Alley, selling potions, medicinal drugs and wares from the Muggle world that they think the Wizarding World could use for their own good as well as potions ingredients and supplies that few other apothecaries in the area have in their stores. They’re happy to have Regulus mooching around the shop as an assistant every now and then, with Regulus happy to not receive any form of payment (Merlin knows he has too much now the Black inheritance has befallen he and Sirius’ vault; Regulus just knows his mother would be turning in her grave if she knew both of them were spending it on the Potter heir).
Over the next few months which turn into slowly flowing years, Regulus finds himself becoming more of a regular at the shop, meaning his visits to the Potters are solely evening and weekend visits which pains him but allows him to watch Harry grow. He thinks his favourite memory of watching Harry grow up is the first time that James and Sirius got Harry on a broom.
Harry had just turned three and Sirius had whipped a kid’s broom out from behind his back as a birthday present for him. James had cackled as Harry clapped his hands and jumped up and down in excitement; Lily’s eyebrows had raised in an entirely unamused way but she and Remus had shot cushioning spells around the living room and allowed James to lift Harry onto the floating broom.
Harry was turning into one of the best flyers that Regulus had ever seen and that was only on a kids’ broom but with James Potter as his father and Sirius Black as his Godfather, that was entirely to be expected. What Harry would be able to do on a full-size broom, Regulus didn’t want to think, but he knew James and Sirius had been discussing (behind Remus and Lily’s backs) how to teach the child to do a Wronsky Feint as soon as he turned 11 and got a full-size broom that could be used on the grounds of the Potter Estate.
Watching Harry start school at the age of 4 had been an emotional experience for all of them: Lily had cried so much that Remus and Sirius had been the ones to actually take Harry into his classroom and meet his teacher before his parents had the chance to, mainly because James had been too busy comforting a weeping Lily outside the school gates. Regulus had turned up as emotional support when James’ massive stag Patronus had turned up at his flat asking him to come and comfort Lily so James could go in and make sure Harry was settling in.
Lily and James had made the decision that it would be best for Harry to start his education at a Wizarding primary school just like the other Wixen children that he would be going to Hogwarts with. And when Harry had come home talking about the little boy with white hair and silver hair who he’d met that day, Sirius had fallen on the floor laughing so hard that he’d cried whilst James had gone on a rampage around the house threatening Lucius Malfoy with every bone in his body.
Sirius was clearly Harry’s favourite person in the world: He was Harry’s first letter to home from Hogwarts, a letter where his words fell over each other in a jumbled mish-mash of excitement and explanation about his Sorting, his dorm and the boys he shared with, the fact that he’d been frowned at for trying to go and sit at the Slytherin table with Draco and a few of the other boys he’d been at school with already, Ron and Hermione (‘she’s a Muggleborn, Sirius! I think Mum’s going to absolutely LOVE her, she loves books and telling me and Ron off for all sorts.’), and what his teachers were like.
Regulus wasn’t at all upset by the ranking that Harry had given him. Third after Sirius and Remus wasn’t bad, considering that Harry now knew their cousins Narcissa and Andromeda through school, along with their extended families, as well as the entire Weasley clan and a whole host of other Wixen families. He held in high privilege the fact that Harry still put him third above the Weasley family, who took Harry under their wing as soon as it became clear that Ron and Harry had become extremely close friends.
Regulus knows it’s because he’s a little too like Lily for Harry’s liking: Regulus was happy to take Harry to the library when he needed it for school, and Lily, Remus and Regulus are the three that Harry comes to if he has any questions about things like school or questions that need serious answers “because Pads won’t give me sensible answers Mum, and I know you and Uncle Reg will!”. But Remus sends him sweets and eventually takes up the DADA position at Hogwarts when Harry’s in his third year and if it’s just so that Sirius and Remus can spend more time with him then what’s Regulus to do other than send Harry all of his potions notes from when he was at Hogwarts? No-one needs to be any the wiser for it but Harry’s potions grades suddenly get better and there are less confusing letters knocking at his window at silly hours in the morning.
He may not be Harry’s godfather but he can sure as hell do just as good a job as Sirius in a completely different way.