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For all that it is James Potter who becomes Sirius’ best friend, it is his younger brother, Harry, who changes Sirius’ life first.
It is a regular day in eight-year-old Sirius’ life. He spends several hours doing his best to ignore his tutor in the morning, sits through a thoroughly boring lunch with his father, and tags along unhappily after his mother on a shopping trip. Then, within a week, everything changes: Sirius and Regulus are invited to live with their grandfather, Arcturus, who had never shown any particular interest in them before.
When Sirius asks why, Arcturus replies, “I received a letter from the young Harry Potter, Fleamont’s second grandson. He saw how you were being treated on the trip and had quite a bit to say on the matter. I arrived at Grimmauld Place to prove him wrong, then determined a better course of action, when I wasn’t able to. Your mother, she has always been this way?”
Sirius looks down. It’s unbelievable, to suddenly be believed. “Yes, sir.”
“And your father?”
Sirius only nods.
“Well. It is only a handful years until you’ll be off to Hogwarts. Until then, you and Regulus will choose your bedrooms here. I expect you to be on your best behavior.” Arcturus smiles. “I am an old man. You must save these pranks I have heard so much about for Hogwarts. And not to worry—I’ve been in touch with your tutor. You won’t miss even a day of studies in the transition.”
Sirius is too grateful to even groan.
Before long, he finds that Godric’s Hollow, and in it the home of the Potter family, is only a twenty minute’s walk from Arcturus’ estate.
“Did it work?” Harry asks. He is a boy a year younger than Sirius, with dark hair and eyes, and a kind smile.
Sirius nods. “We both live with Arcturus now. He’s old, but he’s nice. He doesn’t… he’s not like my parents.”
“Good,” Harry says with surprising force. “No one’s allowed to treat you like that. If anyone is being unfair to you, if something’s wrong, you let me know, okay?”
It should be laughable, the idea of going to someone younger than himself for help. But it’s not, because Harry is so sincere, so determined, and he’s already proved that he can help.
“I will. I promise,” Sirius says, hugging Harry tightly.
And he does.
James is the one who quickly becomes Sirius’ best friend, wading through ponds to catch frogs with him and flying with him on the regular. Harry is less rambunctious, less playful than the two of them, choosing to stick to his parents or older neighborhood kids more than playing with the two of them. An old soul, Harry’s parents call him, has been ever since he was born.
Sirius tests out Harry’s offer of protection with skinned knees and lost objects, and finds help from Harry every time. He goes to him when Regulus vanishes for a day—to visit their parents, which doesn’t go as well as Regulus hoped—and when he finds a bird with a broken wing.
Harry never lets him down.
As the years pass, Sirius welcomes Harry into Gryffindor, and he runs to Harry when his pranks go awry, and he’s in need of shelter or an alibi.
James is his brother, Sirius decides, but Harry is… special in ways that Sirius can’t name. He knows Sirius better than Sirius does himself—knows every wild and wicked part—and he helps him anyway. Every time.
Even when Sirius goes to him, hopeless, with fear that Regulus might join the Death Eaters now that he has graduated Hogwarts. He doesn’t expect Harry to be able to help. Then, a few days later, the Daily Prophet reports that Lord Voldemort is no more, brought down by an unknown force.
Sirius tracks Harry down to his childhood bedroom. He lingers in the doorway, watching James replace bandages around Harry’s chest. Harry’s back is to him, the conversation already in progress.
“No—yes—no.” Harry groans at whatever James said before Sirius’ arrival. “I just want him to be happy. Why is that so hard for people to accept? It’s not—anything other than that.”
“Because three days after Sirius turned to you and said that he was worried about his brother joining Voldemort, we’re celebrating Voldemort’s death.”
“That’s— I had a timeline! It was going to happen anyway. I just moved it up a little. That’s all.”
As Harry continues to detail why defeating Voldemort had nothing to do with Sirius at all, and that he was just trying to do better this time around, Sirius steps fully into the room. James catches his eyes and they share a look. All fondness and hopelessness and understanding, because this is their Harry. Thank you, Sirius thinks, and James doesn’t need legilimency to understand.
“I’ll take over,” Sirius says, taking the bandages from James.
Harry huffs at that, looking over his shoulder. “I told him I could have done it myself.”
“I want to do this,” Sirius says, and finishes the job, taking care with Harry’s injuries. Quietly, he says, “I don’t thank you enough.”
“You don’t need to.”
Sirius looks into Harry’s eyes as he smooths down the bandages. “I do. You said you want me to be happy. I’m always happiest around you.”
“Sirius…”
“Help me, Harry,” Sirius says, a smile tugging at his lips, widening as Harry laughs in reply. “Come on.”
“Yeah, okay,” Harry finally says, and lets Sirius kiss him like he’s wanted to do for years.