Koshiro Chikuma is a character from the 2005 film Shinobi: Heart Under Blade, where he serves as one of the five champions of the Kouga ninja clan chosen to fight in a sanctioned death match against the rival Iga clan. His background is defined by his status as a purely physical fighter. Unlike most of the other ninjas in the conflict who possess supernatural abilities, Chikuma relies entirely on his exceptional skill. He is explicitly noted as having no supernatural powers, instead being the finest user of conventional weapons among the Kouga.

His personality undergoes a dramatic shift over the course of the story. He is initially presented as a cheerful, confident young man who takes pride in his fighting abilities. His motivations are rooted in loyalty to his clan and a desire to prove himself in combat. This character is completely transformed when he is accidentally blinded by Gennosuke, the leader of the Kouga clan. Driven by the bitterness and newfound vulnerability caused by this injury, Chikuma becomes a gloomy and dark figure. This blindness is the central defining event of his character development, changing his relationship with his clan and his outlook on the war.

His role in the narrative is a tragic one. He represents the traditional ideals of a ninja in a world overrun by mystical powers, and his suffering highlights the personal cost of the clan feud. His key relationship is a complicated one with his clan leader Gennosuke, whose actions cause his disability and create an undercurrent of inner conflict within the Kouga. Chikuma's most notable ability is his complete mastery of conventional weaponry, making him a formidable opponent in direct combat despite his lack of supernatural gifts.