Mustard

The Gas User is an unnamed villain affiliated with the League of Villains and a member of the organization's Vanguard Action Squad. He and the rest of his team serve as the main antagonists of the School Trip Arc.

Appearance
He is a skinny teenager with wavy, light colored hair. He wears a black gakuran school uniform along with a gas mask that completely hides his face, attached to two oxygen tanks on his back. He also wears light blue gloves and a green helmet.

Personality
The Gas User seems to bear jealousy and resentment towards Yuuei students, as he constantly mocked Itsuka Kendou and Tetsutetsu Tetsutetsu for being "pitifully simple", and yet be "pampered by the world" due to their school's reputation.

He has also shown to be pragmatic and merciless, having brought a gun to the Vanguard Action Squad's raid on the Yuuei training camp, and shooting towards Tetsutetsu's head with no hesitation.

Field Training Arc
The Gas User began to move after he listened to Stain's declaration.

School Trip Arc
The Gas User observed the summer camp alongside with Himiko Toga, Muscular, and Dabi waiting the orders to strike. Next night, he started the attack spreading his gas in the forest, incapacitating the majority of the students, while he continued to move his gas in the forest. Then, he encountered Tetsutetsu Tetsutetsu trying to attack him from the front, he surprises Tetsutetsu pulling a gun and shooting him in the face, unable to hurt him but breaking his gas mask, then he begins to mock Tetsutetsu saying that simple-minded hardening types like hin just resort to battles of endurance and for that reason they just charge in without thinking and calls him spoiled by the world, while he presses Tetsutetsu with gunshots Itsuka Kendou tries to distract hin from behind and uses her quirk to dispense the gas allowing Tetsutetsu to breathe and catch him from behind with a straight punch, knocking him down instantly, later that night he is captured by the police alongside with Muscular and Moonfish.

Quirk and Abilities
Unnamed Quirk: His Quirk allows him to generate a sleep-inducing gas, which he can control at will. He is also able to sense the moves of anyone who enters the area the gas is spread on. He is not, however, immune to his own gas, implied by the fact that he constantly wears a gas mask when using his Quirk.

Equipment and Weapons

 * Gas mask - Used probability to protect himself from the affects of his own gas.
 * 38 Revolver.

School Trip Arc

 * Tetsutetsu Tetsutetsu & Itsuka Kendou vs. Gas User: Lose