In Christopher Marlowe's Doctor Faustus, written in about 1589-1592, Wagner is identified as Doctor Faustus's servant. In some published editions of the play, Wagner is also referred to as a student, so it's possible that Wagner served Faustus as an apprentice or in an apprentice-like position.
In some early versions of Doctor Faustus, Wagner is also identified as the Chorus. However, there's no plot-based relationship between the two characters. The Chorus doesn't speak at all like Wagner—Wagner speaks in prose, and the Chorus speaks in blank verse—nor does the Chorus refer to Wagner in any of his speeches.
The most plausible explanation for Wagner also being the Chorus is the possibility that in early productions of Doctor Faustus, the Wagner character and the Chorus character were played by the same actor.
Wagner's presence in the play is to serve as a foil to Faustus, to appear in scenes of comic relief, and sometimes to do both at the same time.
In scene two, when two scholars wish to see Faustus, Wagner assumes Faustus's superior attitude and speech, and engages the scholars is witty, mock-scholarly banter. Wagner even throws out scholarly words and Latin words and phrases much like Faustus would, as if he were a scholar himself:
WAGNER. Yes, sir, I will tell you; yet, if you were not
dunces, you would never ask me such a question; for
is not he corpus naturale? And is not that mobile? Then
wherefore should you ask me such a question?...
Thus having triumphed over you, I
will set my countenance like a precisian, and begin
to speak thus:—Truly, my dear brethren, my master
is within at dinner, with Valdes and Cornelius, as this
wine, if it could speak, would inform your worships;
and so, the Lord bless you, preserve you, and keep you,
my dear brethren, my dear brethren. (2.20-34)
Wagner, Faustus's servant, frightens the Clown into being his servant by surprisingly making two devils named Baliol (sometimes "Banio") and Belcher appear. Wagner seems to have learned to conjure devils solely by observing his master, Faustus.
Wagner's scene with the Clown is a classic "clown scene" of Elizabethan theatre, full of comic wordplay wherein one clown, Wagner, out-clowns the other clown, the Clown, who ultimately becomes Wagner's clown and servant.
There's a serious side to Wagner's character, in that Wagner seems to be the only character in the play with any personal relationship with Faustus, although the relationship is not particularly close. Faustus wills his entire estate to Wagner because his relationship with Wagner might be the only relationship resembling a familial one that Faustus has. Wagner also expresses heartfelt concern for Faustus and the way Faustus has lived his life that goes beyond a purely master-servant relationship:
WAGNER. I think my master shortly means to die,
For he hath given to me all his goods:
And yet, methinks, if that death were so near,
He would not banquet, and carouse and swill
Amongst the students, as even now he doth,
Who are at supper with such belly-cheer
As Wagner ne'er beheld in all his life... (Chorus 4)
Wagner is often assigned the speech in the Epilogue. The speech looks to be more appropriate for the Chorus, but the speech nevertheless conveys some of the thoughts and feelings towards Faustus that Wagner expresses in the play:
WAGNER. Cut is the branch that might have grown full straight,
And burned is Apollo's laurel-bough,
That sometime grew within this learned man.
Faustus is gone: regard his hellish fall,
Whose fiendful fortune may exhort the wise,
Only to wonder at unlawful things,
Whose deepness doth entice such forward wits
To practice more than heavenly power permits. (Epilogue, 1-8)