Permeation grants Mirio the ability to phase his body through physical matter.
Mirio can choose to make one part of himself intangible or his entire body. This allows Mirio to smoothly maneuver through solid objects and effortlessly evade incoming attacks.
If Mirio fully activates Permeation, not only does his worn normal clothing fall off, but gravity will drag his body's mass through the ground as well. If he deactivates his Quirk while phasing through the ground or another solid object, Mirio is instantly repelled to the surface due to his own body mass being unable to overlap with any other preexisting mass.
According to Mirio's father, Permeation could potentially lead to fatal results to the user if handled incorrectly. The Quirk is noted to be very hard to control.
Mirio's senses are negated when using this Quirk, as the necessary elements for them to work (light, oxygen, sound, etc.) will also pass through him.
Mirio's Permeation does not extend to other people, meaning he has to be extra careful while handling those he needs to rescue. If someone becomes susceptible to an imminent danger, like projectiles or other hazards, Mirio becomes unable to block them unless he deactivates his Quirk.
Mirio is noted to have mastered his Quirk, having trained vigorously to counteract its major drawbacks.
Despite being aware of the terrible dangers his Quirk could bring to him since childhood, Mirio never once hesitated from his ultimate goal of becoming a hero. Mirio struggled a lot with Permeation, having trouble phasing through a simple plank of wood during his time at middle school. Thanks to his neverending determination though, Mirio was able to learn how to utilize his Quirk well enough to at least pass the U.A. Entrance Exam, thus being accepted into U.A. High School's hero course. Still, Mirio kept facing hardships even as an U.A. student, failing to attain a great result at the U.A. Sports Festival, but his bright and humorous character would grab the attention of Sir Nighteye, who saw him as a potential successor for All Might. Under Nighteye's internship, Mirio achieved perfect control over Permeation, following a rigorous training regimen.
To pass through an object without falling down to the Earth's core, Mirio must undergo a series of specific steps involving different parts of his body turning intangible while others do not. Most people admit that they would never be able to accomplish that successfully in a rush, but Mirio is able to perform it as if it were second nature. He is capable of showcasing meticulous precision on when and where in his body Permeation should be active while performing swift moves. For those reasons, guarding oneself against his attacks in combat is essentially impossible.
Thanks to his rigorous training, Mirio was able to master instant transportation through the repelling property of his Quirk, 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 outside observer, Mirio gives the illusion that he is somehow teleporting, because of his apparent disappearance and reappearance at various spots, which happens almost immediately.
- Blinder Touch Eyeball Crush (ブラインドタッチ 目潰し Buraindo Tatchi - Metsubushi?): 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.
- Phantom Menace (ファントム・メナス Fantomu Menasu?): 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.
- Permeation is similar to the concept of noclipping in video games, and Mina Ashido compares his Quirk to a collision detection glitch.
- Despite light passing through his body while his Quirk is active, Mirio is still completely visible to outside observers.