An explanation is in order:
Naturally I'm aware of the R=U theory, but I don't find the arguments for it convincing. Although at least it tries to answer the big gaping plot hole about Ultimecia's very murky origins.
I find the obvious but vague reasons she hates SeeD unsatisfying; I'd love to see some specifics. Since none are forthcoming I've begun to write a fanfic about her origins.
My idea is that she was unaware of her tragic destiny and joined SeeD to defeat what she would later learn was herself. With her friends turning on her would provide her with plenty of motivation for revenge (not to mention a touch of Oedipal tragedy).
The uniform is based on Balamb Garden with a different color scheme. I gave her a crossbow to refer to the goddess Artemis, who was a renowned archer.