Fumikage's Dark Shadow quirk appears to have a personality of its own, like a second head or a cheeky friend who comes from the shadowy nature of Fumikage's quirk. When Fumikage loses control of his quirk, Dark Shadow takes control of Fumikage and his body by coating itself around Fumikage when there is no light, as bright light is what keeps Fumikage from using the full might of Dark Shadow. While the complete darkness awakens a darker, unruly side to Dark Shadow that Fumikage has trouble controlling. After Hawks teaches Fumikage how he can use Dark Shadow to help him fly, he has Dark Shadow hide parts of itself in Fumikage's cloak in order to shield it from the sun's light. During class training and the sports festival, Fumikage summons Dark Shadow to aid him and his assigned teammates, while for the concert Class A put a guitar play Fumikage has Dark Shadow playing the Cymbals beside him even Dark Shadow wasn't happy the bright stage lights.
During the second battle between Class B and Class A, Tokoyami is seen using Dark Shadow for trying to catch one of the class b's teammates, specifically Shihai Kuroiro, who's got a similar quirk as Tokoyami related to the darkness called "Black". However, Shihai uses his Quirk handling to take control of Fumikage's Dark Shadow and turns it against him, leaving Tokoyami in disadvantage as Kuroiro is seen taking Yuga Aoyama away from Tokoyami and his teammates. He then decides on using "black fallen angel" which is a technique devised under the tutelage of Hawks and said technique could allow him wrap himself up in Dark Shadow's arms and allowed himself to be carried by it, while Dark Shadow could fly by flapping its arms like wings. However, after saving Aoyama and heading on a dark place, Kuroiro reappears and catches Aoyama again as Tokayami's wrapped by Kinoko's mushrooms and being damaged by Manga Fukudashi.
Tokoyami reunites with Hagakure, and after helping her with removing some mushrooms from her, they both see Yaoyorozu's lucky bag thrown in the middle of the air and Tokoyami is seen using Dark Shadow for catching the bag as he's seen carrying Hagakure too. Both notice that the things inside the bag (infrared goggles and three spray cans of ethanol) were support items made by Yaoyorozu herself. Tokoyami is seen with Dark Shadow flying with the infrared goggles on straightforward to where Kuroiro and Kinoko were reunited thanks to the goggles, which allowed him seeing his targets, while Tokoyami uses "Sabbath", a technique in which Fumikage equips Black Abyss (which could equip Dark Shadow onto himself, wearing it like a suit of armor) and uses two shadow claws to strike down Kuroiro and Kinoko in a flash. Dark Shadow then affirms Tokoyami that he has catched Kinoko.
However, Tokoyami overall fails to render Kinoko unconscious, which it gave Kinoko the opportunity to implement mushrooms in Tokoyami's latter's throat and lungs, which it effectively incapacitate Fumikage through asphyxiation. Dark Shadow goes back to Tokoyami while as its seen very worried about Tokoyami's health.
Fans have noticed that Fumikage's Dark Shadow quirk has a personality of its own, and how it sometimes has Fumikage's back or shows possible hidden sides to Fumikage. This has inspired fans to feature Fumikage hanging out with Dark Shadow in their fanart and fics. Some TokoTsuyu fanwork has Dark Shadow teasing or persuading Fumikage to tell Tsuyu Asui how he feels about her, this idea or something similar to it has also been seen in a few other Fumikage ships, like TokoDeku and TokoShoji. The shadow nature of Dark Shadow is also one of the reasons why fans like to pair Fumikage with Shihai Kuroiro, who also has a shadow related quirk.