I thought that Serge was Magus/Janus when I first beat Chrono Cross.It was because of two things:
First - The letter of Lucca to Kid.There Lucca guesses that Janus will find her to protect her(or already found).
Second - In the end of the game(True Ending) there will be the photograph of Schala sitting in the chair.Behind her there is a boy(Janus,I suppose) who looks just like Serge but in Lynx's clothes(is this military clothes?).Though I can't say it is Serge, because you can't see the upper part of boy's face.But isn't there amazing similarity?
But when I played Radical Dreamers, which was the ancestor of Chrono Cross, on SNES, I thought I might be wrong.So what do you think about it?