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 Yuuei's summer training camp alongside Muscular, Dabi, and Himiko Toga, admonishing the former for his impatience and reminding him that there is no need to do anything flashy. When Himiko comments that the situation isn't cute, he assures her that their "shadowy orchestrator" has everything planned out and that there should be sound logic behind it all. The rest of their team arrives soon afterwards, the time almost at hand for their mission to begin.

The next night, the masked villain started his attack by spreading gas via his Quirk in the forest, the first victim being Juuzou Honenuki of Yuuei's Hero Department Class 1-B. He then stood victoriously in the middle of his gas, hands raised to the sky. The Gas User then began to move forward through the forest, the effects of his Quirk following him.

Standing in the middle of his gas, the masked villain paused, sensing that multiple individuals had figured out the intricacies of his Quirk and had started coming for him. Stating that he expected no less from a prestigious school like Yuuei, the villain was otherwise unconcerned, reaching into the folds of his uniform. Pulling out a hidden gun, he goes on to say that no matter how great their Quirks may be, his opponents are still human. As he is in the middle of saying this, Tetsutetsu Tetsutetsu comes charging at him through the gas. The Gas User finishes his sentence by aiming his gun at the reckless student and firing.

The masked villain's shot manages to destroy Tetsutetsu's gas mask, but doesn't inflict any damange on the student due to his Quirk. He acknowledges that his group knew there were a couple hardeners among the students, and that his gun consequently won't work against Tetsutetsu. He goes on to say that it doesn't really matter in the end, as the student will now be unable to escape breathing in his Quirk's gas. The Gas User's opponent charges at him again, with the masked villain shooting him again to no effect. He questions why the student keeps on using the same tactic over and over, saying that if he is going to a prestigious school, he should at least attempt some better strategy, or he isn't even worth killing. The masked villain then aims to his side, firing and foiling a sneak attack attempt by Itsuka Kendou, but hitting Tetsutetsu once more, this time in the head.

The Gas User admonishes the two Yuuei students for their simple plan, explaining that the gas around them is his creation and that he can sense all their movements through it. As he says this, he backs into his gas and conceals himself within it. Tetsutetsu attempts to follow him, only for the masked villain to appear by his side and shoot him in the head once more. The Gas User notes that the student's metal skin seems to be weakening, and wonders how long he can keep that up and not breathe in his gas at the same time. He fires at Tetsutetsu's head yet again, labeling him as a simple hardening type who just rushes in without thinking. He then questions the fact that Tetsutetsu is aiming to be a hero some day, saying that it doesn't make sense how a simpleton like him is praised solely because of his school's reputation. He reloads his gun while saying this and fires two more shots into the prone student's skull. He finishes his speech by stating that Tetsutetsu's privileged situation isn't right, and kicks the prone student, sending him flying a short distance away.

Itsuka then tries to attack the Gas User, only for him to easily dodge it, reminding her that he can detect her every move. She then activates her Quirk, which enlarges her entire hand and manages to strike the masked villain. Knocked back a few steps, he warns her not to get overconfident over such a stupid ability, hiding himself within his gas once more. He is then surprised when she uses her two enlarged hands to rapidly blow his gas away. Itsuka then taunts the Gas User about having no confidence in a fight, because of the fact he brought a gun with him, causing him to target her with it out of anger. While he is focused on her, the masked villain fails to notice Tetsutetsu approaching him from behind until the last moment. Tetsutetsu lands a steel-enhanced punch on the Gas User, shattering his mask and knocking him unconscious. The villain's gas begins to disperse immediately after his defeat.

Later, after the rest of the Vanguard Action Squad succeeded in fulfilling their mission, they left the defeated members of their group behind. The Gas User is then captured by the Police Force alongside 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 to protect himself from the effects of his own gas.
 * 38 Revolver.

Battles
School Trip Arc
 * Tetsutetsu Tetsutetsu & Itsuka Kendou vs. Gas User: Lose

Quotes

 * (To Tetsutetsu Tetsutetsu) "Coming at me like the Terminator, huh? And why're you just charging in like a madman? Give me a break. You're getting a top-notch education at a famous school. At least use some strategy. If not...you're barely worth killing."


 * (To Tetsutetsu Tetsutetsu and Itsuka Kendou) "Bwa ha ha ha! It's two-on-one, so one of you decided to hide for a sneak attack?! Ha ha ha! Simple. Pitifully simple. I'm creating this gas and controlling it!! Every move you make...creates ripples and tells me where you are! There's no hiding from me!! Why's that so unthinkable to you? You're from U.A., so show me something more! I guess I'll forgive you for an attack like that..."


 * (To Tetsutetsu Tetsutetsu) "Hey...aren't you supposed to be heroes someday? It doesn't make much sense to me...how a simpleton like you gets praised because of your school's reputation...you're pampered by the world just because of your alma mater!! Don't ya think it's not right?!"