It is voiced by Yoshimasa Hosoya in Japanese, and by Josh Grelle in the English dub; both of which also voice Tokoyami in the same series.
Dark Shadow manifested within Tokoyami at a young age. The two became friendly with one another, as both learned of Dark Shadow's relationship with light and darkness. Tokoyami used Dark Shadow to enter U.A. High School's Hero Program and the two learn to use their abilities to become heroes. Tokoyami even trains in a dark cave to better control Dark Shadow.
However, one night at the Forest Training Camp, the area is attacked by the League of Villains. Tokoyami's peer, Mezo Shoji, is injured by the villain Moonfish. The feelings of anger and fear cause Dark Shadow to go Berserk and begin attacking everything, with Tokoyami unable to control it. Moonfish is defeated by the enraged Dark Shadow, and Bakugo Katsuki and Shoto Todoroki use their Quirks to create enough light to calm Dark Shadow.
After the attack, the students begin training their ultimate moves, Tokoyami develops a move where he has Dark Shadow cover his body, called Black Ankh.
Dark Shadow is a dark form resembling a humanoid with a bird-like head. It is entirely made up of shadow energy, resulting in it being black and purple in color with glowing yellow eyes. When berserk, Dark Shadow grows larger and more formless and gains glowing red eyes.
Dark Shadow is extremely loyal to Tokoyami and will obey him without question. Most of the time, it is stoic and quiet like Tokoyami himself, but also will act friendly and helpful to its master and his allies. However, as the light level decreases, Dark Shadow becomes more rebellious and violent. If it is too dark, Tokoyami loses control of Dark Shadow, who goes berserk and begins causing mass destruction to everything around it.
Powers and Abilities
Dark Shadow is housed within Tokoyami's body, and is connected via a sort of energy-based "umbilical cord". Tokoyami's Quick allows Dark Shadow to manifest outside. Once out, Dark Shadow can shapeshift to Tokoyami's desires, shield him, and be used for long and close range ramming and clawing attacks. If Dark Shadow or Tokoyami are injured or knocked unconscious, the other will not feel the effects; the two have separate bodies and consciousness from each other. Dark Shadow's power and personality change depending on the light; if it is bright, Dark Shadow will adopt its more docile, yet stoic personality and power. The darker it gets, the bigger and more powerful Dark Shadow will become; however, its personality shifts and it becomes more unruly, rebellious, and violent. If it is too dark, it becomes near-impossible for Tokoyami to control and acts violently and berserk.