When it comes to most of my characters (TOO MANY WHUT), I have a sort of . . . moment of time in their canon that IS that person to me. Something funny, or a moment of character development, or a startling revelation--just something that sticks in my mind as being that character.
Asuna's "moment" comes in volume 7 of the Negima! manga. She's found out that Negi went off into a very dangerous situation without informing her, and when she confronts him about it, he says he didn't want to put her in danger because she's just an ordinary schoolgirl. Asuna is furious and sulks in the beach house for a day or two (they're hanging out during the summer with some of their class by the seaside or something) and later, when Ayaka confronts her about it, admits that she's forgiven Negi for leaving her out of his life for pretty much the first time since they met, but is being stubborn about letting him know.
So how does she let him know? That night she comes to his window in her bathing suit, wakes him up without a word, drags him outside, throws him into the water from the dock, and begins pointing and laughing hysterically at the look on his face. Then she runs up to him, hugs him, begins crying, and explains how worried she was that he'd be hurt and that she's come to care about him so much that she wants him to let her protect him no matter how dangerous the situation.
. . . THAT'S my Asuna. ♥