Gastro

Gastro is one of 3 assassins hired by Akira Takaoka at Okinawa, along with Smog and Grip. He loves the taste of a gun, so much that he uses it as a utensil.

Synopsis
Gastro first appears as an assassin to stop Nagisa and the others from retrieving the antidote from Takaoka.

When encountering part of the E-Class in the theater, he easily spots every single of their locations, shooting at them, though they luckily survive due to taking cover. Though the E-Class feels hopeless, Korosensei comes to the rescue and distracts Gastro with his unique, invincible form, while giving the remaining E-Class members directions to swap positions to confuse Gastro.

Though confused, Gastro finally believes to have located an E-Class student, waiting for their movement. As the "student" moves, it only turned out to be a decoy, made during the attack itself. Chiba and Hayumi work together to disarm Gastro, leaving him to be tied up by Nagisa.

After Takaoka is taken down by Nagisa, Gastro and the other assassins have actually escaped using their assassination knowledge. Though hired by Takaoka, they actually swapped the Takaoka's virus formula with a replica that only works for three hours, but doesn't end up killing the victims. He explains to the E-Class members that losing his reputation of being an excellent assassin would be better off than being worth a bounty for killing a class of students. He then leaves on a helicopter, explaining that he'll kill the E-Class only if they're worthy of it.

Abilities
He has the ability to know where his targets are and their precise movements, making him one of the best marksman to date.