Gas grants the user the ability to generate a poisonous, sleep-inducing gas from their body.
The user is capable of producing a large amount of purple-colored gas, enough to completely fill The Beast's Forest. Anyone who inhales the gas directly will immediately fall asleep and remain that way until the toxin is treated.
The presence and movement speed of anyone within the gas can be sensed by the user thanks to its fluctuations. The gas' concentration is highest at its center and loses potency in lesser affected areas away from the user. The gas swirls in a typhoon, completely centered around the user. Navigating through the gas is also very difficult, similar to a very thick fog.
The gas can be filtered using any sort of gas mask, and lighter concentrations of it aren't as effective. The user is also incapable of manipulating the gas beyond its natural flow, meaning they cannot use their Quirk in combat. If the user loses consciousness, the gas dissipates completely, which suggests that the gas alone has a low potency and that Mustard produces more gas than it dissipates on its own, or Mustard simply changes the properties of the air around him to form this gas, though this is very unlikely. While it has a lot of range, it's still limited, as the miasma didn't cover much more than the forest. The gas can also be blown away easily, as Itsuka Kendo demonstrated, thinning out the gas and making detection through its fluctuations much harder for the user.
Mustard is capable of flooding an entire forest with poison gas, easily boxing in enemies and blocking potential escapes. He usually stays away from his primary targets, best serving as support for ambushing them instead. By keeping his distance and reading the flow of the gas, Mustard prevents himself from being taken by surprise.
The gas is very potent and takes effect immediately, and Mustard can spread the gas quickly to take out numerous people at once. However, anyone using a breathing apparatus can avoid the effects of his Quirk and challenge him in combat. Mustard carries a handgun to compensate for this weakness. He also wears a gas mask, implying that he's not immune to the effects of his own Quirk, though it's possible he wears it for psychological reasons.
- This Quirk works similarly to Somnambulist. Unlike Gas, however, Somnambulist doesn’t have toxic properties, has no direct effect on the user, and isn’t potent enough to put someone into deep sleep.
- Another Quirk that is similar is Poison Gas, which also generates a toxic gas from the user's body that the user can be affected by.