William Winsbury is a student of Gunnerkrigg Court. He used to be a bit of a bully who enjoyed taunting others in order to upset them, but appears to have since grown out of it.
William first appears as the class joker, making fun of Annie, and later also Alistair Kershaw. He also tended to have regular quarrels with the Headmaster's daugther, Janet Llanwellyn. However this has been shown to be a ruse for just as long as it was their way of hiding their romance.
Throughout most of his appearances it was heavily implied he was in a relationship with Janet, but it was only revealed to the other kids in during their hangout in Chapter 34: Faraway Morning, at this point all the other characters seemed to have picked up on it. Word of Tom states (on the Bonus Page of Chapter 57) that the other characters just thought they were messing around, and didn't actually believe them then.
William was also present during the cruise on Cruisebot and helped fight off some of the fingers.
He gets close to asking Annie out in Chapter 57, but is interrupted by Janet, his actual girlfriend. It is then revealed that it was all because of a misunderstanding by other students, which they couldn't get out of without revealing their relationship (again).
His name might be inspired by scottish balad from 18th century, called Willie o' Winsbury, in which a beautiful blonde lad of the same name falls in love with, and impregnates, king's daughter Janet (Janet?).