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When Sirius Black was first sorted into Slytherin, he thought his life had ended as he met his new friend's furious eyes. It was obvious his newly formed friendship with James Potter was over.
He feared he would become everything his parents were, and he didn't know how to handle that. He kept to himself. Despite his house, he didn't trust any Slytherins. Most of them left him alone, too.
One day, he came upon James Potter bullying Severus Snape. He watched, not interfering as Severus fought back. He definitely gave as good as he got. Sirius left the scene as quietly as he had come.
That night he walked into the restroom and saw Severus admiring some bruises. He stopped short, mouth agape. "Did James do that to you?"
Severus looked at him, eyes narrowed as if they were assessing him. "No, my muggle father."
Sirius nodded. "I know that feeling. My witch mother is a harpy. She's also a fan of physical punishment."
After that night, the two of them came to an understanding, if not friendship.
They didn't really hang out with each other as Severus's friendship with Lily Evans was extremely strong, but they became allies, something that was very important in the Slytherin house.
In fifth year, after Severus said something in the heat of the moment and tarnished his friendship with Lily, Sirius was the one to comfort him. That comfort included a heated make out session.
They didn't talk much about what happened, but they continued their relationship, if one could call it that. Neither wanted to define what they had, but they wanted to continue to enjoy the benefits all the same.
Sirius knew Severus's potion skills made him highly desirable, and he was being recruited by the seventh years to serve the rising Dark Lord while they were in their sixth year.
Sirius held on tightly; he hoped he could get Severus to say 'no' to them. Although they never talked about their relationship, he did care about Severus.
Severus started to be unusually cruel to him, and Sirius wouldn't have any of that. "Why are you trying to get me to leave?" he demanded one day.
"You're better off without me. You'd be safer without me."
Sirius's eyes softened. "I don't want to be without you, though," he whispered. "Stay with me. Please."
Severus looked deeply into his eyes as if searching. "Okay, I will."
"Promise?" Sirius asked.
"Promise."
Sirius didn't know if Severus would actually keep his promise, He didn't know if Severus would give in to the seventh years, but he wasn't going to give up on his lover. No matter what, he'd never turn away from Severus.