Keisei Tagami is a central character in the first part of the narrative, serving as the Contracted Monk to the Shikabane Hime, Makina Hoshimura. Orphaned at a young age, he was taken in and raised by Makina's father, which forged an early and profound connection to the Hoshimura family. After the entire Hoshimura family was brutally murdered by a powerful group of rogue undead known as the Seven Stars, Keisei assumed the role of Makina's guardian and Contracted Monk, a position that allows him to provide her with the spiritual energy necessary to sustain her existence as a living corpse. Later in his life, he discovered a three-year-old boy named Ouri Kagami, whom he adopted and raised at the Dai-Rin orphanage located next to the Sekku Temple, acting as an older brother figure to him.

Keisei's personality is defined by a stark contrast between a lighthearted, often immature exterior and a deeply compassionate and principled core. He is known for his unorthodox and sometimes perverted sense of humor, frequently playing pranks on Ouri by hiding risqué posters, figurines, or adult magazines among the boy's belongings. Despite this playful demeanor, he possesses a strong sense of justice and exhibits profound empathy, particularly towards the Shikabane Hime. Unlike many in the Kougon Sect, the secret Buddhist organization that battles rogue undead, Keisei refuses to view the Corpse Princesses as mere tools. He treats Makina with genuine dignity and compassion, openly challenging the sect's discriminatory practices, such as their policy of barring Shikabane Hime from entering temples.

Keisei's primary motivation is to fulfill his duties as a monk and protector. His ultimate goal is to support Makina in her mission to hunt down and destroy one hundred and eight rogue Shikabane, which would allow her to earn entry into heaven and find peace. This mission is deeply personal for him, driven by his past connection to the Hoshimura family and his desire to see Makina achieve her salvation. He is equally motivated to protect his younger brother, Ouri, from the dangerous world of the undead, trying to shield him from its horrors for as long as possible.

Within the story, Keisei acts as a mentor and frontline combat partner to Makina, often engaging in direct hand-to-hand combat with Shikabane while wielding a traditional Shakujo, a Buddhist ringed staff. Despite his skills, he is noted for holding a lower rank of Sho Sojo within the Kougon Sect, which some attribute to his unorthodox methods and his failure to master certain advanced techniques like the "jwadan". His key relationships include his former training partners, Akasha Shishidou and Sadahiro Mibu, though the friendship sours when Akasha defects from the sect. He shares a brotherly bond with Ouri and a complex, deeply caring relationship with Makina, whom he views as a younger sister despite her unrequited romantic feelings for him.

Keisei undergoes significant development through his death. He falls into a trap set by his former friend, the treacherous monk Akasha Shishidou, and is fatally stabbed. In the anime adaptation, he is killed by Akasha and the Seven Stars member Kowaku. With his dying breath, his primary concern is for Makina's fate; to prevent her from reverting to a feral, rogue Shikabane upon his death, he performs a final act to transfer his contract to Ouri. Following his death, the Kougon Sect posthumously promoted him from Sho Sojo to the higher rank of Gon Sojo in recognition of his heroic service and sacrifice. His legacy endures through Ouri, who adopts Keisei's compassionate ideals and takes on the role of Makina's new Contracted Monk.