Faustus's servant, Wagner, is more than just a servant. He's also a bit of a smart aleck who, like Faustus, enjoys the thought of having someone at his beck and call. Let's be honest: who wouldn't?
When Faustus's scholar buddies ask Wagner where Faustus is, Wagner mocks them by responding with fussy, pompous language that would be right at home in a scholarly debate. But while he may mock them, he also wants to be like Faustus, who is, as it happens, a very pompous scholar.
So it makes sense that when Faustus contemplates handing his soul over to the devil, just so he can have his very own magical servant, Wagner has a similar idea. But of course he's not a super fancy smarty-pants like our friend Faustus, so he has to come up with a different plan.
Left to his own devices, Wagner has to rely on good old-fashioned manipulation to finagle himself his own servant. So he hands over two guilders to gain the unemployed peasant Robin as his apprentice. Despite his overall lack of magical prowess, he does manage to conjure two devils to spook Robin into the deal. This tells us he's been watching Faustus closely, and learning from him.
At the end of the play, Faustus wills all his possessions to Wagner, and that's no small deal. Faustus had all kinds of goodies to his name. But why choose Wagner as his beneficiary?
Wagner and Faustus are either closer than they appear in the play or (more likely) Wagner is the only thing approaching family that Faustus has, which is just plain sad if you think about it. If Wagner is the best Faustus can do, well, then that's not really saying much, now is it?
Plus, Wagner's ability to conjure suggests that Faustus's skill is perhaps not as rare as he likes to believe, since his servant can pick it up just by spying on him. And to top it all off, Wagner's use of magic shows that Faustus's dabbling in the dark arts has had further-reaching consequences than he knows, and has spread beyond his control.
So while Wagner is, of course, a character in his own right, much of what he does in the play is to highlight some rather unsavory things about Feisty Faustus.