Technically, he is a ninja. His skill tree outfit screams 'ninja,' and I wouldn't be surprised if other versions of GC were to call him that. What 'theif' that you know besides Lass throws kunais anyways?
But that's just my two cents.
I wanted him to look that way, like as if he were working for the government/kingdom for reconnaissance and assassinations.
Pinkie Pool's archnemesis: Oatmeal.