Korosensei has the appearance of a yellow octopus. He is about three meters tall and possesses a large, bulbous head with a large smile that rarely breaks or moves and small, beady eyes. He uses two tentacles as arms, each having two feelers as fingers, and utilizes the rest of seemingly jelly-like limbs - his tentacles - to walk. He has a pair of hidden nostrils, which make him appear to have four eyes if detected. Due to his constant grinning, he can change his color depending on his emotions.