I read a book some years ago (mid-2000s) which I'd like to find again.
The main character is a (probably teenage) girl who's recently moved with her parents to live on an island (?), or at least a remote region. I seem to remember their move was due to her mother cheating on her father, but this wasn't a big plot point. There were a few chapters consisting of chatroom conversations between her and a friend, in which a mysterious stranger appeared who was visible only to her. His name first appeared as "V", but later changed to "E" after she worked out that his name was Epsilon.
Strange things began to happen on the island, with this girl somehow involved. At one point she went to an abandoned house expecting to find something very significant, and ended up finding ... an old bucket. There were also references to "the Lemon Squire", some sort of secret traditional title; in the end this person turned out to be the local medic. I believe there was a character named Yolanda, but for the life of me I can't remember who she was (definitely not the main character).
Towards the end of the book there's a scene where the main character's loyalties are severely challenged: she has to choose between Epsilon and Yolanda (I think) and isn't sure which side is the "good guys". Eventually she makes the right choice and sides with Epsilon.