Yeah, from what I recollect they were friends when he lived in town. After MC left, he cut off all contact with back home. In the years MC was gone, they both grew up separately and very different personalities. So them not gelling right away, or at all, when they reunite isn't entirely unexpected.
Zach was written well for his role as a heel. The first time through, you see the looks he gives the MC and I didn't perceive it as a festering anger growing... more that his ego couldn't handle flaming out so horrifically when he thought he was all "that" and the jealousy grew. I believe you can see the change in looks he gives the MC as his ego gets the better of him. However Zach met Ethan, that ego and jealousy is what Ethan exploited... probably very easily.
