Jiro Mochizuki is a central figure in the story, an Old Blood vampire with a history deeply intertwined with a great conflict known as the Hong Kong Holy War. He is often referred to by the epithet the Silver Blade, a title earned for his legendary feats during that war. Originally a lieutenant in the Imperial Japanese Navy, his life changed forever in 19th century London when he was seriously injured while protecting a woman named Alice Eve. Due to her love for him, Alice turned him into a vampire, making him the sole vampire she ever created and bestowing upon him a very special lineage known as the blood of the Sage. He is physically striking, with long black hair and dark grey eyes, and his distinctive attire consists primarily of red and black, including a red overcoat and wide-brimmed hat, with a pair of aviator-style goggles often seen hanging around his neck. He carries a long katana with a silver blade, which is a key component of his combat identity.

In terms of personality, Jiro is generally calm, polite, and possesses a mature demeanor. Despite his age and serious nature, he can display a cocky or teasing side, particularly when facing an opponent like Cassandra Jill Warlock or when playfully interacting with his coordinator, Mimiko Katsuragi. His primary driving force is his deep devotion to his younger brother, Kotaro Mochizuki, who is the only other vampire sharing his rare bloodline. Kotaro is the reincarnation of Alice Eve, the progenitor who turned Jiro, and Jiro has a sacred duty as the chosen guardian to protect Kotaro and eventually return the Sage's blood to him. This motivation defines his existence; he reveals that once Kotaro fully matures and absorbs the blood, Jiro will no longer be around, as he is fundamentally Kotaro's protector and destined to be absorbed by him. He shares a close, protective bond with Mimiko, the human coordinator who acts as their escort and link to human society, and his relationship with Cassandra is a complicated love-hate dynamic, as she was responsible for betraying him during the Holy War.

As the story progresses, Jiro comes out of a form of retirement to take Kotaro to the Special Zone, a haven for vampires, where they settle as Mimiko's escorts. He becomes embroiled in a new conflict with the surviving members of the Kowloon Children, the enemies from the past war. Throughout the narrative, his character development is tied to his acceptance of his fate. He declares war on the Kowloon King on behalf of Eve's lineage and sees his duty through to the end, ultimately succeeding in defeating the King again. In the final confrontation, he faces Cassandra, the last of the true Kowloon bloodline, before the war concludes. After the conflict, he lives peacefully with Mimiko, with whom he becomes lovers and has a son, while he awaits Kotaro's awakening. As fated, on the night Kotaro wakes, he enjoys one last moment with his family before being absorbed by his brother, completing his duty. After his death, he is celebrated as a heroic figure who symbolized the potential for harmony between vampires and humans.

Jiro's notable abilities are immense, befitting an Old Blood vampire with the Sage's lineage. He is a master of Shihyen-ryu, a swordsmanship style inherited from his Chinese grandfather, and his physical prowess is far superior to ordinary vampires, allowing him to effortlessly dispatch groups of younger Kowloon Children. Among his supernatural abilities are telekinesis, sometimes referred to as the Hyde Hand, and a form of hypnosis called Eye Raid. He is also immune to the bite of the Kowloon Children, meaning he cannot be forcibly turned into one of them. However, he possesses the classic and severe weaknesses of a vampire: he is extremely vulnerable to sunlight, running water, garlic, and holy symbols like the Bible, to the point where even a little sunlight or water causes his body to smoke and burn. This forces him to rely on an umbrella and move through shadowy areas when outdoors.