Mustard, a teenage antagonist within the League of Villains, operates as part of their Vanguard Action Squad during the Forest Training Camp Arc. His Quirk, Gas, enables him to generate dense clouds of sleep-inducing vapor and detect movement through disturbances within the mist. Lacking immunity to his own ability, he relies on a green gas mask and oxygen tanks for protection, though the gas rapidly disperses once production ceases, restricting its combat longevity. A lean figure with light brown hair, tan skin, and brown eyes, Mustard dresses in a black gakuran uniform paired with teal gloves and a gas mask modeled after World War I military designs. Pragmatic and calculating, he prioritizes tactical efficiency over aesthetics, openly criticizing peers like Himiko Toga for prioritizing style. His arrogance and resentment fuel a belief in meticulous strategy over raw power, coupled with disdain for U.A. High students, whom he views as overprivileged due to their institution’s prestige. He mocks their perceived tactical naivety and resents the acclaim society grants them, justifying his ruthless actions—such as attacking Tetsutetsu Tetsutetsu and Itsuka Kendo with a revolver—as retribution. Despite his Quirk and firearm use, Mustard’s physical frailty and minimal hand-to-hand combat skill leave him vulnerable, with official assessments marking his power and speed as low but technical skill as moderate. During the forest raid, he ambushes U.A. students by saturating the area with gas, incapacitating Juzo Honenuki and engaging Tetsutetsu and Kendo. Taunting their reliance on direct tactics, he boasts of his sensory advantage within the gas. His overconfidence proves fatal when Kendo deduces his dependency on the mask, prompting Tetsutetsu to shatter it, exposing his face and knocking him unconscious. Arrested alongside Muscular and Moonfish, Mustard refuses to divulge League secrets during interrogation. His juvenile status likely spares him from Tartarus, redirecting him to detention. No further background or post-arrest activities are documented, and his narrative role remains confined to the Forest Training Camp Arc. His design and Quirk draw from historical military aesthetics and sulfur mustard gas, cementing his niche as a tactically oriented but short-lived antagonist.