Cygnus

"I'm sorry," Regulus whispered. Suddenly, when Remus looked at him, his calm and collected facade was gone. He didn't look indifferent or angry. He looked like a scared fourteen year old boy. "I just- Mother got so much worse after Sirius left. I w-was in so much pain a-all the time and there was something nice about knowing that I-I was hurting on my own accord. That I was in control for once."

"Reg-"

"I hate Sirius so much for leaving me there," he finally admitted. "I love him but I resent him because if it weren't for him, I'd feel a bit more okay. If he'd just stayed or- or taken me with him. Anything but leave me there with them alone!"

Or

When Sirius runs away to move in with the Potter's, Regulus takes it hard.

His friends set out to prove he's not alone.