At the end of most versions of the game, Guy suddenly attacks Cody.

It was originally believed that Guy beat Cody up because he was jealous that Cody got the girl (Jessica) but in actual fact, Jessica had just called out Cody's name, Cody ignored her and Guy was annoyed at Cody's behavior (The whole point of their quest was to rescue her), so he knocks Cody down to give Jessica a chance to catch up. Furthermore, Guy already has a lover (Rena) and in the game's intro he doesn't seem to know who Jessica is.