WMG / Takamachi Nanoha Of 2814

Negi either has some sort of Glamour or else Nobody Can Grasp His True Form
The main piece of evidence for this is that Nanoha takes him as being a girl. This is not just a case of Viewer Gender Confusion on her part - Negi is a Humanoid Abomination and/or Eldritch Abomination a la a heroic, adorable Slaanesh: everybody sees him in the form that is most appealing to them.