A magus gets all MWP and can punch above its level for a little while each day - a cohort would be at least a couple of levels below the party and could well be a liability with a purely martial class.
If you want a purely martial class anyway maybe a vigilante? They could do the face work and stealth perhaps.
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Vigilante doesn't seem appropriate for the squire role unless their master is a Vigilante, too.
They'd have fewer hit points, true (also true is that that kind of discourages Magi, which are already predisposed to be more fragile than more "purebred" warrior-types), but other than that there's no reason they'd be a liability no matter their class. They'd just have to be more careful.
As such, you may well want to create a character who favors fighting at range (while still not being an idiot savant in that regard). Gunslinger, Ranger, Fighter, and Skald would all be fine choices (archery-oriented Paladins can be pretty vicious too, so I've heard, but then there's the matter of alignment compatibility to consider). Another very interesting possibility would be Occultist.