Alex is partially based on Constantine, true, but he’s partially based on Xander’s incomplete UK knowledge, hence the fact that I can mess around with accents a whole bunch and still be strictly accurate. (There are, however, some phrases and facts about Britain that he’s never learned, so magic.) Alex can actually cast some spells; Xan can’t get around magic without causing something to go wrong. Xan keeps trying to force Alex to not be an intellectual, but he doesn’t have the same preconceived notions as Xander.
He’s less likely to listen to ‘obvious’ things (‘guy code’, etc) and more likely question them. In a way, he’s Xander’s conscience, the thing that speaks up and makes him feel guilty (and possibly lash out defensively), only he can do so safely since he and Xander need each other. Not to say they don’t argue, but they also need each other.