Igor is Dr. Frankenstein's other assistant, the one with the bug-eyes and the hump on his back. His grandfather worked for Frederick's grandfather, so being an Igor runs in the family. We have to wonder if this Igor is getting paid enough, though.
Instead of assisting, Igor pretty much trolls the doctor whenever he can. He makes fun of his mispronounced name ("Well why isn't it Froderick Fronkensteen?"), insists his own named is pronounced "Eye-gor", and pretends not to notice he has a hump. (Not only does the hump move around, but it's hollow to the touch.) Do we have any evidence that Igor is doing this all on purpose and isn't just a complete idiot? Not really, but "Call it… a hunch."