Sirius was bitter and cold and at times incredibly cruel. Sirius was not the friend who rooted for James and Lily to get together, the one who smiled whenever he saw them holding hands by the fire, quite the opposite in fact. Sirius would sabotage every attempt James made of impressing Lily. When they finally started dating, Sirius would pick fights with Lilly and would consistently make James pick between them. One night James went out on a walk with Lily instead of playing exploding snap with Sirius, and for three days after Sirius gave James the silent treatment all while scoffing whenever Lily opened her mouth.
It ended with a huge fight where James demanded that Sirius give Lily some ‘bloody respect because, honestly mate, I have never been this happy before, and if you actually gave a shit about me, you would understand that.’ Sirius looked at James and saw the look in his eye–a look he had ever seen directed at Snape. His heart clenched, and he swallowed hard before giving a half-hearted apology, knowing that if he didn’t that the damage to their friendship would be irreversible.
Sirius was impulsive. When he told Snape about the Whomping Willow, he wasn’t thinking about the implications. He just imagined Snape’s expression being face to face with a werewolf, and that Snape would never try anything with them ever again. Sirius was thinking about the time Snivellus sent that sectumsempra curse at James, and how him being ripped apart by one of them would be perfect justice.
When Sirius found out Lily was pregnant, he went on a rampage. He broke two of his chairs and smashed all of his plates, causing his muggle neighbors downstairs to call the cops for a domestic. He turned into Padfoot before anyone showed up though, and he stayed in dog form for a week. During that week he was feral; he wanted control and he did so by hunting anything that he could find. He snarled at smiling children that approached him, reminding him that his best friend, his brother, was abandoning him. Tossing him to the side like he was nothing, all for what? A screaming infant that he could have prevented had he not mucked up the contraceptive curse?
With each rabbit he tore apart, the more thoughts flashed in his head. James would never go on another mission for the Order. He had lost his only family for a drooling brat who would only further isolate James away from him. James would soon forget about him as he turned into this bumbling ‘family man’ who refused to take risks and sat around all day reading the paper and smoking a pipe.
It took days for Sirius to get the blood out of his hair, charms and all.
Sirius was reckless and chased adrenaline. Once in awhile, particularly if something happened with Regulus, he would suggest this outlandish prank that not only endangered his life, but others as well. There was one night in fifth year that Sirius dragged James out of bed to watch the stars atop the Astronomy Tower. Once they got there, Sirius just turned to give James this empty smile as he ran towards the edge and jumped off.
James pulled out his wand and within milliseconds had levitated Sirius back on his feet. Which only lasted for a moment before James pounced on him, pinning him to the cold marble ground. Sirius still had that smile on his face as James growled, demanding what in Merlin’s name he was on about.
Sirius just shook his head and told him that his Mum had written him. He was officially out of the will. ‘Not yet blown off the wall, but close enough.’ Without thinking James wrapped his arms around him, and held Sirius as tightly as possible as Sirius sobbed for what seemed like hours against his shoulder.
Sirius could be vicious, calculating and incredibly aggressive. But Sirius knew this about himself, and he would sit at night at the edge of his bed and think about these impulses and the fire brewing inside of him. He believed that this was simply genetic. He was a Black, and he was slowly morphing into his destiny. This anger and callousness, this was everything he knew. How could he be any different?
James always had a way of sensing these thoughts, and would silently get out his bed and sit next to his best friend. No words needed to be said, because James’ presence alone reminded Sirius what he already knew: I’m a Potter by choice, and choices are what make you who you really are.