Chikuma Koshiro is a character who appears in the anime and manga series Basilisk, and he has a specific role in the live-action film adaptation Shinobi: Heart Under Blade. In the context of the film, Koshiro is presented as a member of the Kouga clan, a significant departure from his original Iga clan affiliation in the source material. He is a young man characterized by his long, messy black hair and notably bushy eyebrows, with two small scars on his left cheek. His appearance is further defined by his traditional ninja attire, which includes a black sleeveless kosode over a netted shirt and black hakama. After losing his eyesight in battle, he begins wearing a black mask that covers the upper half of his face.

His personality differs markedly between the original manga and the anime adaptation. In the manga, Koshiro shares the ruthless and arrogant demeanor of his master, Tenzen Yakushiji. Conversely, the anime portrays him as more reluctant to engage in battles where his opponents are at an overwhelming disadvantage, showing a sense of chivalry that his master considers a weakness. Despite these differences, he is consistently depicted as deeply loyal, though this loyalty becomes a source of intense internal conflict when his master's actions put the woman he is devoted to, Oboro, in danger. His motivations are driven by his devotion to his master, Tenzen, and his deep, potentially romantic feelings for Oboro, his childhood friend. He is fiercely protective of Oboro, but his obligation to follow Tenzen's orders often places him in a difficult position between the two most important people in his life.

In the story of the original Basilisk, Koshiro is selected as one of the ten Iga ninja to fight in a deadly proxy war against the Kouga clan, a role that puts him in direct opposition to his friend's love, Gennosuke. A pivotal moment in his development occurs when he attempts to use his special ability against Gennosuke. When Oboro intervenes, a combination of her and Gennosuke's powerful eye techniques results in Koshiro being blinded. This event transforms him, forcing him to rely on his other senses and don a black mask that becomes a key part of his design. This loss of sight ultimately contributes to his downfall, as his inability to adapt to his blindness makes him vulnerable in a later confrontation. Key relationships define his character arc. His relationship with Oboro is central, marked by his complete devotion and his silent, unrequited love for her. He is the apprentice of Tenzen, a cruel and powerful ninja, to whom he feels a powerful sense of duty despite internal conflicts. Akeginu, Oboro's bodyguard, harbors romantic feelings for Koshiro, but he remains almost completely oblivious to her affection. In Shinobi: Heart Under Blade, his relationships are altered, notably serving as a Kouga clansman who grows frustrated with his leader Gennosuke's pacifism and decides to confront the Iga alone.

Koshiro possesses several notable abilities. His primary and most distinctive technique is Kamaitachi, which allows him to create destructive miniature vacuums with his breath, capable of shredding opponents. He is also a highly skilled wielder of twin kama, or hand scythes, which he uses with near-perfect accuracy in both close-quarters combat and as thrown projectiles. In the live-action film, his abilities are re-imagined; he is portrayed as a weapons specialist who wields shuriken and kunai attached to chains and can project hundreds of wooden spikes at his enemies.