I literally binge watched every episode of Teen Wolf over the summer and I don't remember Stiles writing letters to Lydia? When was this? What were they for? I gotta know!
In episode 3x18 Lydia and Aiden are in Stiles room trying to figure out where he is, and Aiden sees a frame with a tree drawing in it and asks Lydia if that’s the one she drew. She tells him to put it back, which then Aiden responds with he likes you a lot, doesn’t he?. Lydia then says that maybe he just likes the drawing a lot, Aiden then says
for Lydia referring to the note that is attached at the back of the photo frame.