Sirius Black didn't always have long hair.
That fact would have surprised anyone who didn't already know. Sirius' hair looked so good, and so natural, that no one could be blamed for thinking he came out of the womb with a full head of silky black hair curling under his chin. Harry was surprised when he finally saw pre-Azkaban photos of his godfather.
But it was true. Until he went to prison, Sirius Black had a continually-shaved head.
Of course, he had to keep it cut in what Walburga deemed a respectable haircut when he was home, at least until he was sixteen and didn't care about offending his parents anymore. But he could charm his hair longer for the holidays, so as soon as he got to Hogwarts and met James Potter, his hair was gone quicker than you could say 'razor.'
Sirius liked to do it the Muggle way—it felt better. Every week he'd run the clippers over his head, the Marauders clustered around him, trying to distract him so he'd mess up and razzing him for his lack of facial hair. (Even years later, he couldn't grow a proper beard.) He kept the habit up even after Hogwarts (and after he swore never to see his mother again). Baby Harry loved to run his chubby little hands over the soft stubble. It was nice, too, to have short hair as Snuffles. His fur never got matted, and any mud was easily brushed off when dry.
When Sirius first entered Azkaban, he traded for razors on the inmates' black market. But as the weeks, months, and years dragged on, and as his sense of self got eaten up by the silence and the cold and the dementors, Sirius shaved his head less and less often. Eventually, his hair dangled below his bony shoulders. He was so lethargic that he didn't even notice the unwashed hair making his shoulders break out in pimples.
Living in Grimmauld Place hardly helped. The tapestries, the dust, the mould, the lingering sense of his mother's disapproval, and his curtains of hair all contributed to his sense of being suffocated. Most days Sirius thought he could scream for hours—that is, if he could dredge up even a shred of energy, and if he were allowed out of the house. Looking at himself in the mirror in the morning was a constant reminder of all the things wrong with his life, and nothing more so than the hair.
So, eventually, Sirius got Harry to buy him a set of clippers. He was planning to barricade himself into the toilet alone, but Hermione got wind of his plan and decided to help. She draped a towel around his shoulders and cut great chunks of hair with kitchen scissors until his hair stood up at all angles. She picked up the clippers and was about to shave the first long stripe when Sirius lifted his hand to take the clippers.
"Wait, I'll do it." Sirius took a breath and settled his shoulders before running the clippers all over his skull. Greasy clumps of hair rained onto the towel. Hermione stepped out into the hallway to give him privacy. After he was finished, Hermione popped her head into the bathroom, closely folllowed by Ginny.
"You'd look even cooler with eyeliner and an earring," Ginny declared.
Sirius grinned. "Give me a makeover, Ginny."
Ginny directed Sirius to clean and trim his nails. With Hermione's help, she lightly plucked the space between his eyebrows and gave him a facial complete with cucumbers over his eyes. She couldn't give him an earring right then and there, but she did carefully line his eyes with the unopened black eyeliner Molly had given her a year ago.
They proudly showed Sirius around the kitchen at the Order meeting that night. Remus dropped his fork with a clatter and had to excuse himself, blushing. Harry clapped Sirius on the back and asked Ginny if he could get a makeover, too. Tonks had a great time metamorphising into an exaggerated version of Sirius with raccoon eyeliner and doing bad impressions of him to all the Order members.
But that night, Sirius stood in the bathroom, one hand pressed to the glass. "I'm back," he said softly, "I'm back." After sixteen years, he finally recognised the face in the mirror as himself. "Take that, Mother," he said, a slow grin spreading across his face.