Katy was a rather "mysterious" character and not too much said about her. At one point she says to Sebastien " Don't worry, There's a lot I don't tell Lily" What do you make of her?

Katy seems like the all American cheerleader type with cool blonde hair and not much to say. Her murder was a shock since the reader did not get to know her very well. Was this on purpose by the author?

Katy seemed like a pretty normal person to me. She didn't seem to particularly like or trust Lily,. It's certainly normal to not like someone.

Several characters in the book believed Katy had had an affair with Sebastien, due to one passing comment she made about a relationship she'd had that Lily wouldn't like. Despite Katy's apparently wholesome appearance it's surprising they believed she do something that seems so out of character for her. Why the comment about her not telling Lily everything, though, I wonder? Was this a red herring, to put doubt into the reader's mind?

I thought it would have helped the story to know more about Katy. She was "mysterious" to the point of not being a full character.

Yes, Katy was mysterious. Not surprised she did not tell Lily everything and was often seen as "close-mouthed" in the story. They met for the first time when they were both assigned to live in the same house as exchange students. From the start you can tell these two would not be "friends". Often times you get roommates you do not like - but since you live in close quarters for a short period of time, learn to exist with each other. Maybe people reacted to Katy based on her looks and mannerisms which is not necessarily who she was.

I suspect that Katy was not well defined intentionally. The fact that the reader knew so little about her helped heighten the mystery of what happened to her. There are too many hints about Katy having a wild side for it to be mere coincidence.