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Sirius knew about Kneazles.
No one in his family had ever kept one, of course — no one was of solid enough character to not drive the poor creature mad — but Frank and Alice had had one when they’d invited him (and James and Lily and Remus and Peter) for dinner that one time. They’d joked lightly about the Kneazle being defective because she seemed to have it out for Peter.
They should have listened.
After what happened, Sirius vowed to always trust a Kneazle whenever he encountered one.
So, when, as he was trying to hunt Peter down at Hogwarts (thank Merlin for that newspaper photo), he met what must have been a Kneazle-cat crossbreed, he knew to show submission at the first hiss.
He didn’t want to risk anything.
He wasn’t sure of his character, all things considered, and the creature would doubtlessly be a better judge of it than he would.
It took some time to earn the creature’s trust, which Sirius supposed was fair. He did have murderous intentions, after all, no matter how justified those intentions might have been. He was hardly a bastion of selflessness, or even goodness.
He wanted Peter dead for his own reasons as much as anything else. He wanted to mete out justice himself.
It took weeks to get the Kneazle-cat (whose name Sirius couldn’t quite glean) to understand him, and longer still to trust him. He could tell the creature knew Peter wasn’t a real rate; that he was bad and wrong and full of malice.
The creature also knew that Sirius wanted to capture the not-rat and keep him away from Harry and his friends. (Sirius had managed to sort out that Harry’s friend Hermione was the Kneazle-cat’s human, which felt like an odd stroke of luck.)
So, between them, they built a tentative trust.
When Frank and Alice’s son (Neville; his name was Neville) dropped a scrap of parchment containing the passwords to Gryffindor Tower, the Kneazle-cat fetched it and provided it to Sirius — with a clear message of “You’d better get it right.”
To his credit, Sirius very nearly succeeded.
He would have succeeded if Ron hadn’t woken up; it would have been. He could tell the Kneazle-cat was disappointed in him, but the two of them kept trying to achieve their mutual goal.
It was a mutual goal, of course: to get rid of Peter and keep their loved ones safe from the rat. The Kneazle-cat knew as well as Sirius that the rat was a threat, and that Sirius was, somehow, the only one who believed him.
When this was over, Sirius would have to educate Harry and his friends about Kneazles and their trustworthiness.
(He did not think about Frank and Alice’s Kneazle, nor about what must have happened to the thing; Frank had once laughed that his mother detested animals. Surely she’d gone to a good home after they’d been driven mad, right?)
The chase was, to pardon the pun, the most frustrating game of cat and mouse Sirius had ever played in his life.
For someone who spent his late teens and the first year of his twenties hunting down Death Eaters (off the record, of course; he’d failed the psychological evaluation to become an Auror — thanks, Mum — and had only been able to work through the Order after that), this particular Death Eater was proving especially slippery prey.
Part of it was that Peter knew the castle every bit as well as Sirius did — and he could fit in spaces Sirius never could, even in dog form.
Part of it was that the rat lived under Ron Weasley’s protection (Ron was clearly blissfully unaware that he’d let a thirty-three-year-old man into his room; while Sirius could safely say Peter’s intentions were not nefarious in that particular way, it was still weird, and it was still too close to Harry).
No matter what Sirius and the Kneazle-cat tried, Peter was somehow one step ahead of them.
(Especially frustrating as Peter wasn’t nearly as clever as Sirius — had Azkaban dulled his mind more than he’d thought? It was possible and terrifying.)
Finally, Sirius managed to capture him — and yes, he had to catch Ron Weasley, too, but Sirius would never actually hurt the boy — and the Kneazle-cat was right there with him. Or, rather, he appeared after Sirius shifted back to his human identity, which he’d never once shown the creature, for fear of scaring him off.
It would have all been perfect if they hadn’t been interrupted.
Harry really was James and Lily’s son, and something about that broke Sirius’s heart just as much as it made him proud. James and Lily would have been proud of him.
It was Peter’s fault they couldn’t be.
Peter’s fault they were dead.
Peter’s fault Sirius had spent twelve long years in Azkaban.
If they’d remembered that it was the full moon…
No; the important thing was that they’d exposed Peter. Even if Sirius had to go on the run, Harry and his friends now knew who Peter was, and what to do if they ever found him.
He never would have done it without the Kneazle-cat’s help (he learned the creature’s name was Crookshanks, which was somehow fitting). Finally, Harry and Hermione and even Ron seemed to believe that Crookshanks had had the best intentions, no matter how confused they may have been at first.
Sirius was still going to catch the filthy rat someday, though.
(Maybe Harry was right — maybe murder wasn’t the answer — but capturing him and turning him over to the Ministry was surely within the realm of acceptable actions, wasn’t it?)
Sirius hated being on the run. It was all Peter’s fault, and there was nothing Crookshanks, for all his earlier aid, could do to fix that.
Still, when Crookshanks came to Number Twelve a year later, Sirius thought they still had a good understanding between them. He gave Crookshanks special treats when the others weren’t looking to thank him for his friendship.
He wouldn’t be here without him, after all, and even living in Number Twelve was preferable to Azkaban, or the Dementor’s Kiss.
Maybe this way he could stay in Harry’s life after all.
That was really what James and Lily would have wanted, and he wanted to honour their memories. Maybe one day they could get that home together, once Peter was captured and the war was over.
At least he hoped so.