the way the series begins, goes on continuing and ends – one may begin to get to believe himself of being one of the three lead characters, thats always the case. the most interesting character developed (& kept secret) for most of the series is Severus Snape (played magically by Alan Rickman).
Interestingly, this character has it all.
He is in love with Lily Potter, and cannot tell her somehow.
Gets Bullied by James, Sirius Black and Remus Lupin.
Pretends to be a follower of the Dark Lord but isn’t
and most importantly – the best friend of Albus Dumbledore.
He, in short, acts as a double agent for Voldemort, but honest to Dumbledore’s intentions. Wants to save Harry for the good that James once saved his life… and infact Harry being Lily’s son.