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.)
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.