Regulus and Sirius' relationship changed drastically after the latter ran away from their childhood home. They were distant, they didn't understand each other as much as they did before. Regulus was, —of course—, reasonably upset, seeing how free his brother became after leaving him in that hell alone. So much so he became angry with Sirius whilst the older black was completely oblivious.

And you, as a good boyfriend to Regulus, tried to smooth things over, to put it simply. You became a messenger boy, delivering letters from Regulus to Sirius and the other way around, you never read the letters of course, you respected their privacy. But the letters didn't exactly smooth things over.

One day, when you had a class without Regulus, and Sirius, by the time you found them, they were at each other's throats, fighting and cursing at each other. You did successfully break the fight off, guiding a fuming Regulus away, towards his spot by the black lake, sitting down under one of the trees and hugging Regulus' shoulders with arm whilst the other ran through his hair, whispering reassuring words, trying to calm your boyfriend down.

Regulus was silently crying in your shoulder, biting down on your muscles to muffle his sobs. Things were going so well with Sirius but one stupid sentence destroyed everything he wanted to rebuild once more, making Regulus feel helpless even more than before.