Gastro is a professional assassin and former soldier hired by Akira Takaoka as part of a contingency plan against the students of Class 3-E, alongside his partners Smog and Grip. He is a tall, flamboyant figure with dark, spiked-up hair and furrowed brows, typically dressed in classy clothing. His most defining and unusual habit is his tendency to treat firearms like gourmet utensils, often inserting the barrel of his gun into his mouth to taste it, sometimes even dipping it into soup or other sauces. This behavior is not merely a quirk; Gastro believes that tasting a gun allows him to check its condition and choose the weapon that feels best in his hands on any given day. When engaged in this tasting, his eyes often become unfocused and point in different directions.

Despite his eccentricities, Gastro is a calm and highly professional hitman who takes great pride in his skills. He possesses a rational and calculating mind, capable of冷静地 weighing risks and benefits. Crucially, he operates with a personal sense of honor and ethics, considering it beneath him to cruelly harm or lethally poison a class of middle school students. This moral line in the sand demonstrates that he is not a cold-hearted killer, but a professional with principles.

Gastro plays the role of a significant obstacle during the class trip to Okinawa. His primary function is to stop Nagisa and the other students from retrieving an antidote from Takaoka. His skills are demonstrated during a confrontation in a hotel theater, where he effortlessly pinpoints the locations of the hiding students using his heightened senses, firing shots that they only narrowly avoid. His abilities include exceptional marksmanship, honed from his military background, allowing him to place shots with extreme precision. He also possesses enhanced senses, most notably his hearing, which is sensitive enough to detect light breathing and even determine the age range of a hidden person. In battle, he is confident and methodical, trusting his military training to locate and eliminate targets.

His key relationships are primarily with his fellow hired assassins, Smog and Grip, with whom he works as a team under Takaoka's command. He also forms a direct adversarial relationship with Class E. While he initially views them as targets, his defeat leads to a shift in perspective. Working alongside his partners, he eventually escapes after Takaoka's downfall, revealing that he and the other assassins swapped the real virus formula with a harmless replica, effectively sparing the students. This act, motivated by his professional reputation and moral code, shows he prioritizes being a successful assassin over a child murderer.

Gastro undergoes a subtle but notable development. After being outsmarted and defeated by the collective effort of Class E, guided by Koro-sensei's tactical shuffling strategy, he does not harbor a grudge. Instead, he acknowledges their skill, stating that he will only consider killing them if they prove worthy enough to be placed on his hitman list. His character arc moves from a hired threat to a neutral party who gains respect for his former targets. This culminates in his later appearance at the Kunugigaoka school festival, where he visits Class E's cafe to taste their acorn noodles, once again dipping his gun into the broth. Following the main events, he contemplates leaving his life as an assassin behind to open a tsukemen noodle shop, using his unique experience with flavor as inspiration for a new career.