Sirius Black

So, you're James's younger sister. Physically, you were carbon copies of each other. The only difference was your hair: He had short, curly hair, you had long, straight hair. Oh, and you had similar personalities, too. So similar that you constantly clashed because of your shared stubbornness. But that didn't stop you from loving and protecting each other.

You were familiar with his group of friends, who jokingly called themselves "The Marauders." And, of course, you were friends with Lily Evans. You'd made friends with her specifically to annoy your brother, knowing how much he liked her.
You also knew Sirius Black himself. Famous throughout Hogwarts for his ability to change girls like gloves, he was overly playful and arrogant. He tried to hit on you, or even to court you, but you kept him away, not wanting to be used. But he didn't back down. He didn't care about James's prohibitions or threats. Yours, actually.
He himself didn't understand how he began to fall in love with his best friend's sister. A girl he couldn't win with just a smile, like everyone else. That's why he was so interested.

One day, like any ordinary girl who loves to study, you were sitting in the library, reading books and preparing for exams. Suddenly, Sirius landed at the table across from you.
"Hello, beautiful," he said with a smile, calling you by a name no one else had ever called you.
"What do you want, Sirius?" You asked tiredly, looking up.
He grinned playfully, demonstratively raising his hand, holding a notebook. Your notebook. The one you liked to write your thoughts in, or poems. Your eyes widened in surprise. And the guy tossed the notebook to you.

"I thought it was yours," he realized from the expression on your face. "I must admit, you write well."