Black Star is a central character who serves as a ninja and a meister at the Death Weapon Meister Academy. He is the partner of Tsubaki Nakatsukasa, a weapon who can transform into various ninja tools like a kusarigama, shuriken, and a katana. As one of the academy's top students, he is a young man with bright blue, spiky hair styled to resemble a star and a star-shaped tattoo on his shoulder, marking him as the last surviving member of the infamous Star Clan. His clan was a family of assassins who were wiped out by the very academy that later took him in as an infant, a past he acknowledges without holding a grudge.

His personality is famously loud, arrogant, and exceptionally confident. Unlike a traditional assassin who relies on stealth, Black Star prefers to make a flashy entrance and announce his presence, driven by an intense desire for attention. He constantly declares that he is a "big" man destined to surpass God, a goal he pursues with relentless physical training. This egotism is balanced by a genuine care for his friends, a surprisingly mature understanding of his clan's fate, and an honorable nature that refuses to take undeserved credit. His partner, Tsubaki, is his perfect foil; her gentle, patient, and easygoing nature allows her to tolerate and support his showboating, making them a highly effective team.

Black Star's primary motivation is the pursuit of power and self-improvement to achieve his goal of transcending divine beings. This drive pushes him to his physical limits and makes him one of the most athletic students at the academy. However, his obsession with power is a double-edged sword. It leads others to fear he might descend onto the path of becoming a kishin, an evil demon god, similar to his father. His role in the story is as a key field agent for the academy, working to reap the souls of evil humans to prevent the birth of new kishin. He is a crucial part of the main trio of meisters, providing raw strength and head-on combat prowess. Key relationships that shape him include his rivalry and friendship with the god-like Death the Kid and his repeated confrontations with the samurai Mifune, who serves as a critical figure in his maturation.

Throughout the series, Black Star undergoes significant development. Initially, his arrogance and hunger for power make him short-tempered and uncooperative in team battles, and defeats can shake his confidence to the point of losing his abilities. His battles with Mifune force him to confront a fundamental choice between the "Path of the Warrior" and the "Path of the Demon." Through this struggle, he matures profoundly, learning to temper his egotism and follow a more honorable and focused path, eventually being recognized as a "Warrior God". By the end of the series, he is acknowledged as one of the strongest characters, with Death the Kid admitting that Black Star had surpassed him.

In terms of abilities, Black Star is an incredibly formidable fighter even without his weapon. He possesses immense physical strength, speed, and stamina, able to perform feats like running one hundred meters in less than six seconds and enduring punishing workouts. His natural combat instincts are so sharp that he is described as a "heaven-sent warrior". A signature technique is Soul Menace, a form of self-resonance where he channels his own powerful soul wavelength into a physical strike to damage an opponent internally. With Tsubaki, he can perform Soul Resonance to execute a variety of named techniques, such as Speed Star for enhanced velocity, Trap Star to bind enemies, and Shadow Star to manipulate shadows. After mastering Tsubaki's most powerful form, the Uncanny Sword, he gains access to devastating attacks like the "Zeroth Form: Masamune" techniques. He can also tap into a state called Madness Release, which grants a tremendous boost in power at the risk of his own sanity.