Khnum is a pure ability and allows Oingo to mold his face to disguise himself as anyone.
Khnum is a Stand that confers the ability to change shape to its user, Oingo. Thus, it doesn't have an appearance or a personality.
The Stand represents the Egyptian God, Khnum, the god of rebirth and creation, alluding to Oingo's ability to reshape his body in any way he sees fit.
Khnum is a Stand with no combative ability, whatsoever, but is useful for approaching people multiple times without them becoming suspicious. Thus Oingo manages to nearly assassinate the Joestar Group and was foiled only by bad luck.
To do so, Oingo can mold his face as if it were clay to modify it. He cannot change his clothes but can alter his own hair, doing so to match Jotaro's hat at one point. Oingo claims he can freely undo his alterations, although he is never seen doing so.