Jet

Jet always were their gasmask. They say that they aren't able to breath without it. No one has ever seen them without the mask except for their roommate and "leader" Xian.
He knows what's under that mask.
But he won't tell anyone.

Jet can change the "filter" modules with other modules, aslong as they have atleast one filter/breathing module to breath. Jet always has a bag full of modules like gas, smoke and "food"-modules since they even have to eat with the mask on.

(^Bio is in work)