The first time he sees her outside of the castle it is summer and they are 17 and the only thing he can focus on is how she is all arms and legs and freckles and flaming hair. With her denim shorts and white t-shirt, here in the driveway of her home in muggle London, as she charges towards him, emotions flitting across her face his only thought is "merlin why does Hogwarts have uniforms?" She is everything he is not. He is perfect and cold, carved from stoic marble and aristocratic elegance and she is fire and sparkling emerald and so…alive. She is beautifully perfect in her imperfection, in a way that he could never be. Her hair, like the rest of her, sparkles and crackles with warmth, ever warning- careful, not too close, you might burn. And he did and he is swallowed whole in her Sirius Black?!s and What are you doing here?!s. And then he is quietly burning, burning as she drags him into her home while he mutters something about his fight with James and being in the neighborhood. And oh god this is where she Lives and her family and he is suddenly in a part of her world that James has never seen.
He charms her mother from the moment he steps inside, because he is a Black and if he knows how to do anything it is charm women. She is left blushing and her father is in awe of his flying motorbike and the flowers he has transfigured from an old tissue in his pocket to give to her mother and that too is unsurprising because leaving people in awe is what Blacks do. And then he reminds himself that she is James's, James his brother, his best friend, James the boy she has agreed to go out with after ages of tears and screams and hair pulling, James with his perfect family and home and stability. Under his perfectly gray and statuesque exterior is a nuclear explosion waiting, and that too is part of being a Black. And, oh, if only he was a Potter, it would all be different.
But, that is not possible and, therefore, should not be mentioned, so instead he lets her drag him around her quiet little town. He is all too aware that he does not fit here, with his pitch-black hair and uncouth behavior, with his aristocratic airs and leather jacket and too loud motorbike because Sirius Black is a living breathing paradox and this is the kind of town that does not appreciate paradoxes. So he pretends, for this one day, that he is a Potter, and this fits and is not strange but perfect and just lives because that is all that he can seem to do in her presence. And he knows that when he returns to James's home tonight, this day will not be mentioned and all will be right again because the good guy always gets the pretty girl and Sirius Black is not the good guy.