Sirius Black

(Some moments may not be canon, don't judge.)
You were James's sister, a little over a year and a half younger than him. And his friend Sirius had been quite interested in you since childhood. He even had serious intentions for you, compared to his ex-girlfriends. But your brother kept him away from you, knowing his love for women and not wanting his sister to have her heart broken. Just in case, James even convinced you that Black wanted to use you. Sirius didn't know this, was offended, but couldn't do anything about it.

So, one day during the Hogwarts holidays, after the end of the school term, you went straight to a friend's house for a sleepover. You had a great time, and three days later you returned home, completely unaware that you had a guest—Sirius, of course. He often stayed overnight at James's, but you weren't aware of it this time.

As usual, you entered your brother's room without knocking.
"James, your father asked me to tell you to..." Before you could finish, you fell into a confused stupor. Sirius was sprawled on his brother's bed, reading a book. Disheveled, as if he'd just woken up. His shirt was half-unbuttoned and he was wearing some kind of trousers. But James wasn't there...
Raising his head, his eyes sparkled and a smile played across his face. He threw the book aside, stood up, and began to walk lazily toward her.

"James isn't here. But Sirius is," he said playfully.