Gastro

Gastro is a professional hitman, one of three assassins hired by Akira Takaoka in Okinawa as a contingency plan against the students of Class 3-E, alongside Smog and Grip.

Gastro takes genuine pride in his craft and, despite his reputation as a dangerous killer, holds to his own code of honor, refusing outright when ordered to lethally poison an entire class of junior high students. He also has one distinctly odd quirk: he considers the taste of guns exquisite and is frequently seen with his pistol jammed in his mouth, occasionally dripping with soup.

Gastro first appears as an obstacle blocking Nagisa Shiota and the others from retrieving the antidote from Takaoka. Confronting part of the class inside a theater, he easily pinpoints every student's location and opens fire, though they narrowly survive by taking cover. Just as the situation looks hopeless, Korosensei himself intervenes, distracting Gastro with his invulnerable form and buying the students enough time to regroup and outmaneuver him.