Dark Shadow grants Fumikage a shadowy, monster-like being from within his body that he can materialize and de-materialize at will.
Dark Shadow effectively acts like Fumikage's personal guardian, functioning as a living appendage that follows all of its master's commands. It is sentient, capable of speech and loyally protects its host, regardless of the state it finds itself in.
Dark Shadow is always connected to Fumikage's body through an "umbilical cord" (as described by Shihai Kuroiro), even when it moves far away from him. However, Fumikage does not appear to share the same physical sensations as Dark Shadow, such as pain and weakness, or vice-versa, meaning that incapacitating just one of them will not prevent the other from acting out. It is somehow able to communicate with Fumikage telepathically, as well.
Dark Shadow appears to possess the ability to change shape to some extent, as it can apparently extend the length of its arms and neck if so desired. It's said to be made of "dark energy", despite being a solid entity that can interact with the material world, but said energy has a limit, and Dark Shadow will retract into Fumikage's body when said limit runs out. Certain actions can make this energy drain out faster, like making Dark Shadow take too many opposing hits or having it distance itself too much from Fumikage's body. To recharge Dark Shadow, Fumikage needs to keep it shrouded from any source of light.
Dark Shadow's personality and offensive power depend on the surrounding brightness levels. Under strong light, Dark Shadow is small in comparison to when it is engrossed by darkness. Dark Shadow is docile when exposed to light, making it easy for Fumikage to control. However, it is also weaker and therefore less dangerous for the opponent in a fight.
Dark Shadow's personality under the influence of darkness is the opposite of when it's exposed to light, becoming far more powerful and vicious while attacking anything that moves or makes sound indiscriminately. The drawback is that Dark Shadow is also more unruly and rebellious, being more difficult for Fumikage to be able to control it efficiently.
According to Mezo Shoji, Fumikage's negative emotions (such as regret and indignation) can intensify this version of Dark Shadow, giving it enough power to defy Fumikage's commands and gradually take over his body with its expanding darkness. Once this happens, Dark Shadow is able to force Fumikage to move around against his will, leaving Fumikage with no control over his body.
Fumikage primarily fights from a distance, ordering Dark Shadow to move towards the opponent and fight on his behalf. Whenever he needs to defend himself, Fumikage lets Dark Shadow handle the opposing impacts for him. Dark Shadow also helps Fumikage with mobility, giving him or his allies support for climbing structures if necessary. Dark Shadow mostly attacks by stretching or growing both of its arms, using them to grab or slam the adversary while Fumikage stays out of danger. This grants Fumikage a huge advantage in one-on-one battles, to the point of being viewed as almost invincible.
Despite his weakness towards the light, Dark Shadow maintains extraordinary defensive skills, as it can still block light-creating Quirks, such as Explosion or Electrification, with some efficiency. Under little to no light, Dark Shadow becomes a large and ferocious beast with incredible offensive abilities. One of Dark Shadow's claw attacks can rip through dozens of trees with ease in this state. Moonfish, a veteran villain, ended up completely overwhelmed and defeated by Dark Shadow's powered-up form, having his own defenses obliterated in a single strike.
After his Hero Work-Studies with Hawks, Fumikage figured out a way to utilize Dark Shadow to simulate flight. As Dark Shadow is always floating and capable of holding its host, Fumikage becomes able to move through mid-air by letting his Quirk wrap around his body with its arms and carry him around.
- Like several other Quirks, Dark Shadow has its own signature sound effect: "CREEP."
- The relationship between Dark Shadow and Fumikage could be in reference to the Marvel character Venom and his commonly associated host, Eddie Brock. Both share bodies with another being, both Venom and Dark Shadow are sentient beings, both care for their host/Quirk user, and both can become uncontrollable. The only difference between the two being Venom can separate from its host's body.