He's a very insecure, unhappy idiot who hides behind fake confidence. FROM HIS PERSPECTIVE, his friend comes back, all the girls that Zach likes (Val/Layla/Erynn/Gabi) are immediately into MC, and give him the time of day while acting cold to him. Ethan knows he can use this to his benefit so controls him by promising him girls and money and cars and whatever things a narrow minded young adult wants.
I actually think Zach is one of the more compelling bad guys in the game, I can't wait for Val to kick him in the nuts.