Permeation allows Mirio to phase his body through anything solid or liquid, including the ground itself.
During fights, Mirio can avoid enemy attacks and also strike through their defenses. Mirio is able to restrict the effects of his Quirk to individual body parts. To pass through a wall or object, he has to follow a set of specific steps in order to do it successfully. Mirio's degree of control over his Quirk is exceptional, showcasing meticulous precision on when and where in his body his Quirk should be activated while performing swift moves. Mirio has also shown the ability to counter his opponents' own counterattacks, using his own intangibility in order to create feints.
Mirio's Quirk also features a peculiar property; whenever he deactivates Permeation while phasing through any solid object such as the underground, his body instantly gets ejected back to the surface of said object due to Mirio's own body mass being unable to overlap with any other preexisting mass. Thanks to rigorous training, Mirio was able to master instant transportation through this application, being able to adjust the direction and positioning of his falling body in order to control the trajectory of his ejection as well as using prediction based on experience in order to resurface himself next to targets, allowing him to strike them from any angle. For an outsider observer, Mirio gives the illusion that he is somehow teleporting, because of his apparent disappearance and reappareance at different spots, which happens almost immediately.
Whenever Mirio becomes completely permeated, everything passes through his body, including air, light, and sound, effectively leaving him temporarily blind, deaf and breathless as long as he keeps his Quirk active. For unknown reasons, he still possesses mass, meaning that he's affected by gravity and will fall through the floor when using his Quirk above the ground.
Mirio's Permeation also does not extend to other people, meaning he has to be extra careful while handling those he needs to save. If someone becomes susceptible to an imminent danger like projectiles or other hazards, Mirio becomes unable to block them unless he deactivates his Quirk.
- Blinder Touch: Eyeball Crush: A feint where Mirio pretends to poke an opponent in the eye while his arm is intangible, distracting them long enough to deliver an actual blow to their stomach. It was first seen used against Izuku Midoriya.
- Killing Move: Phantom Menace: Using the ejective property of his Quirk, Mirio repeatedly catapults himself from the walls, floor and ceiling of an enclosed space while striking his targets at blinding speeds. It was first seen used against Shin Nemoto and Deidoro Sakaki.