Friends believe in friends, especially when they have nothing else.
I had friends once, strange though it may seem for one of my kind. Indeed, I had three of them. They were closer than family: my faithful confidants, loyal allies, trusty companions and partners-in-mischief...and crime. Of all things, of all people in the world, these were the dearest to me, who was shunned, even feared by all the wizarding community save a few hardy souls more often considered mad, and with good reason, whatever my friends said. They were the sort of friends that could not be bought for any price, or so I thought.
For a brief, deliriously happy spell, even amidst the fear and uncertainties of that time, I could count four, and hope for another addition in the child of James and Lily. I could almost imagine I was normal.
Then I had none.
Dead, dead, dead, as good as dead. Of what use tears and recrimination, anger and hate, when you, who had stolen the others from me, were the last, the only one still living - all I had left? And sentenced to Azkaban for your sins, to be the prey of Dementors, our little childhood joys and triumphs their food, as good as dead. It seemed a terrible loss then; I was that lonely.
But I retained sufficient sense, and not enough hope, that I left behind such memories as I could not live with, to return to my long forgotten way of life, before I had such friends.
And you escaped Azkaban; I dared not imagine how. For a few desperate moments I could believe you would come to end it, finish me off as you did the others. I might have been happy. But it was too much to hope, and I was relieved, returning my focus to the sensible business of survival. Harry's survival.
The last, the only, my only.
I believe you. Of course you were not the traitor - it was Peter, who was so long dead it hardly made a difference. Had you wanted to kill Harry, you could have done it more easily while he lived unaware with his Muggle relatives, defenceless. Had you wanted to kill Harry and beard Dumbledore in his fortress at Hogwarts, you could have done it more easily in his first year and placed yourself at Voldemort's right hand in the process. I believed you did not betray James and Lily to Voldemort, just as I believed you when you regretted your failure to kill Peter.
I believed you. You needed no excuses, I could make enough for us both. I was no less culpable in their deaths than you, for in spite of all you and James and Lily and Peter said of my nature and the wolf's, I could not be trusted even to hunt you down as you deserved, and Peter died alone.
I believed you. No less dark than the Dementors, you feared them not, they found you kin. The Dark Arts you had learnt at Voldemort's feet found you a way into Hogwarts past Dumbledore to Harry, not your past with the Dark Creature you had befriended in your misspent youth.
I believed you when, laughing, you said you had killed James and Lily, just as I believed you when you wished I had had killed Severus under the Whomping Willow, and believed it was a joke , merely a willful, unthought jest, not that I was a tool - never a weapon - and never asked if you sent him to his doom or mine. I believed you just as I did when you swore to me you would find a way to accompany me even though my transformations, and swore my dark nature would never come in the way of our friendship.
You must have known it before I did, always closer to the wolf-moon you have never seen (and I pray you will never see), than I ever dared be in your unregulated lunar fury, closer to the wolf I could never admit to myself in the form of your feral familiaris, embodied ill-omen. The wolf understood you better than I did myself, and loved you for it, my fellow would-be killer, and how it hated James that night he stole my prey all those years ago. Your pet Moony was a Death-Eater before Voldemort was a thought in his mother's girlish dreams of family. There can be nothing to forgive between us.
We know each other too well, Sirius, and I could never fault you for trusting me no more than I trusted you. I believe in you as you do me, dearest friend in all the world.