Charles V's knight is skeptical of Doctor Faustus and does not want to see him perform his magic. Faustus makes horns appear on his head in return for his skepticism and snide remarks. (In the B-text, the knight is named Benvolio and has a slightly expanded role, attempting to exact revenge on Faustus by killing him.)
A Knight at Charles' Court Character Timeline in Doctor Faustus
The timeline below shows where the character A Knight at Charles' Court appears in Doctor Faustus. The colored dots and icons indicate which themes are associated with that appearance.
...eagerly asks Faustus to prove his skills in magic by performing a spell, though a knight (the same character as Benvolio in the B-text) at court is skeptical of Faustus. Faustus... (full context)
The skeptical knight doesn't believe that Faustus can bring Alexander forth, and leaves, not wanting to be present... (full context)
The spirits leave, and Faustus asks for the emperor to call the skeptical knight back to court. The knight re-enters and now has horns on his head, the result... (full context)