Regulus Black was 5 years old the first time he had to choose. Sirius had once again annoyed Mother but unlike every other time where she simply yelled, this time she pulled out her wand act cursed him. After the fact he thinks that he could have, should have done something, helped Sirius. Instead, that night he went into Sirius’s room and left his favorite teddy.
Regulus had quickly learned that he couldn't help Sirius when mother cursed or hexed him. Instead, every night after it happened he would sneak in and leave his teddy with Sirius. He knew it was stupid and didn’t help Sirius and that he should’ve done more. But he didn’t, and he won’t.
When he is 7 Regulus breaks a vase playing with Sirius. It’s probably priceless and old and he can’t imagine what Mother will do to him. But when Mother comes in, Sirius tells her that he did it, that “it was ugly and he was doing her a favor” she hit him with a curse that leaves him bleeding out then calls Kreature to come in and clean him up enough that “no marks will be visible”
When Regulus is 10 Sirius goes to Hogwarts and leaves him behind in that house. Then he goes and gets himself sorted into Gryfindor and Mother goes ballistic.
When Sirius comes back for winter break all he talks about is his new friends that he replaced Regulus with. A boy named Peter Pettigrew who is quiet; Remus Lupin, another boy with an odd name (regulus thinks his is much better though since wolf wolf is a dumb name) like them who likes to read but is odd and tends to disappear every month; and finally James Potter who ‘Reg would really like’ as if he could like the boy who took his brother away from him.
Winter break is the first time Mother uses a new curse on Sirius, they were at a dinner with Mother’s side of the family and Sirius of course acts out, why can’t he just lay low instead of poking the bear. When they get home she hits him with it, it’s like he’s being tortured, Sirius is screaming and crying and writhing on the ground and begging for her to stop, and for the first time, Regulus steps in. He yells for her to stop. And then he’s hit and it’s unimaginable and horrific and he wonders how anyone could do that to their own children. He doesn’t go into Sirius’s room that night for fear he’ll be cursed again. Sirius is cold to him for the rest of the break so Regulus spends his time with Kreature.
When Regulus is 11 he too goes to Hogwarts. The sorting hat takes 5 whole minutes to sort Regulus. It says that he is brave enough to be in Gryffindor and certainly smart enough to be in Ravenclaw, but Regulus knows that he has to be in Slytherin. He has no choice. He tells the hat as much and after a minute of convincing it, it shouts “SLYTHERIN”. Mother will be pleased even if he is not, he secretly thinks being in the same house as Sirius would be nice. He tries to make some friends in Slytherin anyway.
Regulus is 12 when he makes it onto the Slytherin quidditch team, he gets the snitch every game that year, even if their team loses because their keeper is god awful. When he is 13 he sees a boy in the locker room and he feels a pull in his gut that he knows he shouldn't feel, it’s unnatural, so he chooses to push it down and ignore it.
Regulus is 14 when he is in defense against the dark arts and the lesson seems eerily familiar, on the board it says “unforgivable curses” and the professor is talking about the cruciatus curse. Suddenly Regulus is 10 and Sirius is on the floor and so is he and the pain doesn’t stop. He tries to tell himself it can’t be the same no matter how similar the curse sounds to what he was hit with. Mother wouldn’t do that to them, this is illegal. Sure she can be cruel but she wouldn’t, she would get arrested. Except she wouldn’t, and she didn’t. Regulus tries to ignore the rest of the lesson.
When Regulus is 15 Sirius and him come home for break. Siris is once again misbehaving at the party and mother curses him with what he remembers as the imperious curse and Sirius behaves perfectly (although for the first time Regulus misses Sirius’s misbehavior). Sirius of course manages to throw it off and mother is furious. They go right home and it’s the worst it’s ever been. He has never seen Sirius so weak. It’s not right, Sirius is the strong one. So Regulus, in a rare moment of bravery, throws himself in front of Sirius. The pain is all consuming, so much so that he can barely hear Sirius going to the floo and yelling “Potter manor” of course it’s Potter. Add he lets go and simply lets the pain take over.
In April Barty manages to drag him and Evan to a Gryffindor party. Of course, Potter is there and he’s obnoxious and arrogant and loud, and hot. He is unfairly hot and Regulus can’t stand it. So of course, when Potter comes over and calls him Reggie and talks to him, it’s only polite that Regulus responds, and when he steps closer so is he to stop him. And he can’t bother rationalizing or justifying when James comes even closer and leans down and kisses him because it’s just so good. And so they keep meeting through the school year in various empty rooms James somehow always manages to find and Regulus, for the first time he can remember, is happy.
When regulus turns 16, his parents make him take the mark. James begs him not to, pleads that he leaves and comes to his house. But Regulus knows he can’t leave, he’s the heir now and his parents won’t stand to be without one and he’s not brave enough. So he stops responding to James’s letters over winter break, a clean break is for the best for both of them. So Regulus takes the mark and leaves the only happiness he’s ever known so that Sirius can keep it.
Over that summer Regulus has nothing and no one so he reads and he listens. Being the most noble and ancient house of Black, many death eater meetings are held at their house. Regulus heard the meetings and he hears the dark lord boasting about how he cannot be killed because of his great power. So Regulus reads. One of the very few perks of living in this ancient hellhole is the amount of dark tomes they have, Regulus reads until he comes across something called a horcrux. It makes the caster immortal as long as the object is intact for the small price of splitting your soul.
In his seventh year Regulus studies, it’s excessive even for him, Barty and Evan notice, his grades slip, his quidditch performance is steadily declining, and it’s all for nothing, he is no closer to discovering what The Dark Lord’s horcrux is. Until Winter break, when Kreature seems off. It feels wrong but he demands Kreature tells him what happened, then he knows.
He knows what he’s doing is idiotic and that he’ll probably die, so Regulus Black chooses to be brave, he’s went along with what his parents wanted his whole life and he just can’t anymore. So he borrows some bravery and he writes out a note even Sirius would think is dramatic and he puts on James’s quidditch jersey and he makes Kreature take him to the cave.