Fumikage Tokoyami is a student in U.A. High School's hero course, placed in Class 1-A. Known by his hero name, Tsukuyomi, he is a striking figure due to his appearance, possessing the head of a black bird, complete with a hooked tan beak and thin red eyes. Despite his avian head, his body is that of a normal human, initially slender but becoming more muscular as he develops through rigorous training.

Tokoyami is characterized by a reserved, serious, and highly focused demeanor. He presents himself as a creature of the dark, often employing archaic, dramatic speech that can be perceived as intense or unsettling to others. Despite his dark inclinations, he is fundamentally noble, valiant, and deeply humble. He is acutely aware of his own limitations and readily accepts advice to improve himself. While he tends to be quiet and can be socially distant, he becomes more sociable and supportive when working within a team, showing great trust and gratitude toward his allies. He has little interest in fame or idolatry but possesses a quiet pride that dislikes being underestimated. His emotional control is critical to his success in battle, as intense feelings like rage or indignation, especially when triggered by harm to those he cares for, can have dangerous consequences on his Quirk's stability.

Tokoyami's primary motivation is to master his powerful but volatile Quirk, Dark Shadow, and to become a hero worthy of the title. He takes his training seriously and shows immense determination, spurred on by mentors like the Pro Hero Hawks, whom he deeply respects and feels a sense of responsibility to represent. His role in the story is that of a consistently reliable and powerful combatant among his peers, often ranking highly in school events and proving instrumental during major confrontations with villains. He has earned the respect of powerful figures like All Might and is frequently deployed on the front lines.

His key relationships are built on mutual respect and trust. He has a strong friendship with Izuku Midoriya, whom he respects as a leader, and has worked effectively alongside Tsuyu Asui, demonstrating excellent synergy. He also shares a bond with Mezo Shoji, whom he accidentally injured during a Dark Shadow rampage but later reconciled with. While he finds Katsuki Bakugo's fighting style savage, he holds no ill will and is grateful for his help during a critical incident. His most significant relationship is with the Pro Hero Hawks, who takes him on as an intern. Under Hawks' tutelage, Tokoyami not only refines his combat skills and develops new techniques but also forms a profound emotional bond, risking his life to save his mentor during the Paranormal Liberation War.

Tokoyami undergoes significant development, transitioning from a student who fears his own Quirk's power in darkness to one who learns to control and weaponize it. Early on, Dark Shadow could rampage uncontrollably in total darkness, causing Tokoyami to plead with his friends to flee from him. Through training, advice from All Might, and the direct mentorship of Hawks, he gains greater mastery. His physique becomes more muscular, directly addressing his previous weakness in close-quarters combat. He develops several powerful super moves, including Black Ankh, which equips Dark Shadow as a suit of armor to mitigate his physical vulnerabilities; Black Fallen Angel, allowing him to use Dark Shadow for high-speed flight; and Ragnarök, a devastating technique that unleashes Dark Shadow's colossal power to clear paths or trap opponents.

Tokoyami's notable ability is his Quirk, Dark Shadow, which manifests as a sentient shadow monster that can emerge from his body. This entity has its own personality and is capable of speech. Dark Shadow's strength, size, and level of control are directly dependent on the surrounding light levels. In bright light, Dark Shadow is weaker and smaller but much easier to control. In darkness, it becomes exponentially stronger, durable, and massive, capable of tearing through trees and overpowering formidable villains. However, this immense power comes at the cost of control, as Dark Shadow is prone to becoming feral and rampaging based on its own will in the dark. Tokoyami's training has focused on mitigating this weakness, allowing him to maintain command even in near-total darkness. He uses Dark Shadow for a wide array of purposes, including powerful physical strikes, defensive shields, enhanced mobility for both ground and aerial movement, and even in rescue operations.