Mendel has no one habitual form. He cannot be assumed to be humanoid.
Like most genies, he is often to be found wearing various appendages of unusual function with which he is experimenting (biological radio-wave receptors, ceramic electrical armouring or poison strips, anyone?). He can resemble a Frankenstein's monster of sorts, if Frankenstein had had access to the listings of Collic's Modern Bestiary.
Mendel is friendly and generally non-harmful, though he does enjoy the odd irregular practical joke. He fits some of the classic stereotypes of a geek, although he cannot speak Klingon.