Disclaimer: J.K. Rowling owns Harry Potter. I don't.
Su was sorted into Ravenclaw in all of two seconds. She'd hardly just sat down when the Hat bellowed, "RAVENCLAW!" So she promptly stood up again, and blushed as the entire roster of Ravenclaws eyeballed her on the way to the table.
Su spent a lot of time alone. Sitting by the lake drawing or reading or writing. In the winter, holed up in some cosy undiscovered cranny. When she did hang around people, it was usually with Luna, or Amy. Luna's dad ran The Quibbler, and she was content in doing and believing things everyone else thought were really fucking weird. Amy, who was pretty well just normal, was obsessed with Arithmancy, thought Su suspected Amy was just as obsessed with Professor Vector.
Su was muggle-born, which was one of the many things that made her "different." (Or, as Su's Auntie Bev liked to say, "free-spirited.") No one else at Hogwarts read old Muggle poetry, or listened to weird obscure bands. Su loved old second-hand clothes. She kept her black hair fairly short, though shaggy. She wore black horn-rimmed glasses, which her mom disdainfully said covered up her pretty face. Brown almond-shaped eyes, pale skin, a smattering of freckles -- average, not pretty, thought Su.
Su wished, sometimes, for long red hair like Ginny Weasley's. Ginny was pretty. Full lips and immaculate teeth, long eyelashes and delicate little ears. Ginny was tall and willowy, with long, pale fingers, her pale skin sprinkled liberally with freckles.
Su always figured she and Ginny would get along. Ginny was delightfully snarky in their DADA class. The girl was brash and gutsy, and she knew all the best hexes long before they ever learned them in class.
Things Su knew about Ginny:
1) She was a Weasley, and she was friends with Harry Potter.
2) She liked Quidditch, DADA, and swimming. Su had spotted Ginny swimming in the lake several times when the weather was particularly warm. And she'd stride out of the water in her pink tank suit with red polka dots like Venus on the half-shell, those long pale limbs shimmering in the sun, her swimsuit clinging to her body.
3) Her birthday was December 11th. Su had overheard Ginny mention it to someone in class.
By the time they were fourteen, Amy talked about boys pretty much nonstop. Michael Corner and Blaise Zabini. Cedric Diggory (may he rest in peace) and Harry Potter. Dean Thomas and Ernie MacMillan. Luna had a thing for Sirius Black for a while. ("He's a brilliant musician!"). Su thought boys were okay, in an abstract sense. Blaise Zabini had a pretty mouth. Ernie MacMillan's hair was nice. But in her dreams it was always lounging around and eating grapes in an endless pile of cushions with Fleur Delacoeur, or reading over Cho Chang's shoulder, smelling her hair, or, best of all, exploring the bottom of the lake with a green-skinned mermaid Ginny, with impossibly long red hair that swirled around Su's arm, or against her calf, or brushed her stomach.
When it came right down to it, when she'd pulled the curtains around her bed, stealthily cast that silencing charm, and pretended it was someone else's hand questing between her legs, what she thought about most was Ginny's perfect rose-red mouth, or that time Ginny had worn that thin tight t-shirt with no bra on that hot day in Hogsmeade, or that faint dark triangle you could see between her legs when she emerged, dripping, from the lake.