Peter
Peter and Sirius's relationship is often complicated. They were just so... different.
Sirius is a leader. He liked being loud, to take up space and make people notice him. He enjoyed playing pranks and messing with people. Sure, he had a temper that often exploded(Remus had once said that he had the temperament of a sleep deprived artist. Sirius knew that he is absolutely right on both the artist part and the temperament. His friends didn't know he loved painting. Just another secret.) and liked seeing how little he could work and still ace every class. It's a fun but dangerous game. Sirius lived for that adrenaline that resulted from balancing on that thin knife's blade. It is part of the reason why he played Quiditch. And part of the reason McGonagall is worried about him.
Peter is very different. He is a follower. He liked being in James' shadow, and not being noticed. He liked riding on decent scores and being a simple helper in a prank instead of one of the main masterminds. He didn't love that blast of adrenaline rush that both Sirius and James lived for at every Quiditch match. He didn't balance on a knife's edge, he stayed safely in one spot.
Part of Sirius respected him for that. He is always more grounded, if not as grounded as Remus is. Peter is also the best at emotions out of all four of them. He can tell when someones sad or angry and is amazing at calming them down. Another part of him is frustrated with Peter for that exact reason. He had so much potential, if he only tried a little harder.
Ok, maybe Sirius is being unfair. Peter had a good heart, he just got on Sirius's nerves so easily. That isn't to say that Sirius didn't care about the boy. He did, he just had a temper. A temper that Peter (although unintentionally) flared. Repeatedly.
It is also a little weird that Peter snuck around so much. Sirius only knew this because he very rarely slept all night, and as a result he often saw peter either slipping out of the dorm or slipping back in.
Today is one of these nights. Sirius had fallen into a fitful sleep and woken up after yet another night mare (Sirius sees himself standing in his family’s mansion, trying to shield his brother from his mother’s wrath. Sirius sees himself as a five-year-old, his mother performing the Cruciatus Curse on him for the first time. Sirius sees himself arguing with his mother over something. Sirius sees himself screaming in pain as his mother yelled at him for being a Gryffindor. Then the scene changes. Sirius sees himself backing away from the werewolf in front of him, too terrified to transform into a dog. He sees James as a stag putting himself in between the werewolf and Sirius, giving him just enough time to transform. The scene changed again, and Sirius was back in his family’s mansion as his mother yells at him. He sees his mother cast Diffindo on him, making cuts appear up and down his arms. He sees himself riding on the train for the first time, hiding many scars and just hoping for a fresh start.
He sees himself safe with his friends at Hogwarts. He sees himself turn to look at Peter, and he sees Peter shift and change until he's back at the mansion standing in front of his very angry mother. He hears his mother shout that he will never be able to escape the House of Black.).
Once Sirius had finally managed to yank himself from his nightmare, he sees Peter's faint shadow against the curtains of his canopy bed. He hears the door shut softly and Sirius wars with himself for a few seconds before finally shoving down any remaining fear and slipping out of the dorm.
He follows Peter's faint silhouette through the castle. With every turn, Sirius gets more confused. He's memorized their map of Hogwart's passageways (both well known and secret), and as far as he knows their isn't anything of significance down this particular hallway.
Again, he shoves down his emotions and continues ghosting after Peter as silently as possible. Eventually, Peter stops at - a portrait of a bowl of fruit? What the heck?
Peter reaches up and - tickles. One. Of. The. Pears. He has officially gone mental Sirius thinks just as the portraits opens and Peter slips through the opening. Sirius pauses, staring at the spot that his friend just disappeared through. Then he shrugs. He's never been afraid of the unknown. Whats the worst it could be?
Sirius opens the portrait (thinking of how ridiculous this must look) and slips through the opening.
He very nearly dies. Death by laughter is a very real thing. Sirius doubles over in laughter as he sees the room that the portrait has led to. The kitchens. Every night, Peter has been sneaking down to the kitchens. Sirius should have predicted this.
Peter whirls around, a piece of chicken halfway to his mouth, and his eyes widen as he sees Sirius standing in front of the entrance way, practically dying of laughter.
"What the heck?!?" Peter sounds a little more than indignant At his friend's antics.
'I'm going to die, this is precious!" Sirius gasps through peels of laughter. "I can't wait to tell James!" Slowly, Peter's faces cracks into a grin. Then he starts to laugh as well. The two boys laugh for a little while, before looting the kitchen for snacks. Once their stuffed (and Sirius has stuffed his pockets with chocolate for Remus) they begin to head back.
'Hey, Sirius?" Peter asks hesitantly.
"Yeah?" Sirius responds, flashing him a grin.
"Could you maybe.. not tell James about thus? Like, I don't mind you telling him about where the kitchen is, but maybe not about, you know, this?" Sirius just shrugs.
"Sure, I won't tell James. If he asks, I couldn't sleep so I was wandering the castle and accidentally stumbled on the kitchen. I promise I won't tell either of them." Sirius gives Peter a smile, then pulls him into a one-armed hug. peter laughs, trying to pull away.
James never finds out about their midnight snack runs, though he does give Sirius an earful about not wandering the castle in the middle of the night and not getting enough sleep. Occasionally, when Peter sees that Sirius is having a rough day, the two of them slip out to get snacks. Remus, being the absolute genius that he is, finds out at some point and eventually joins them.