Takamachi Nanoha of 2814/WMG

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. Hence why everybody is whatever for Negi.