I"m actually terrified that they might make this a movie because Hollywood would completely screw up the fact that this love triangle exists because each one loves the other. It's not really about Tessa choosing between Will and Jem.
William is the 16 year old prince of Ortencia. He's rather cold and detached, but he tries to be kind and interesting. He likes to stay in his own head, take long walks by himself, and he's very determined. He's only been in the school for three years, but he's a fast learner. He has suspicions he might be gay or bi, but he's told himself he'll figure it out another time, and it's certainly not something he wants to volunteer to discuss with anyone.