Eishiro Yabuki is a student at Seidoukan Academy, one of the six schools in the academic city, and he is a first-year high school student. He serves as the roommate and classmate of the protagonist, Ayato Amagiri, and is often seen as a cheerful and outgoing source of information. His outward persona is that of a lively and somewhat mischievous member of the school's first newspaper club, always seeming to know the latest rumors and trends. However, this friendly exterior hides a much more complex and secretive true nature.

Beneath the surface, Eishiro is a highly skilled operative. He is a member of the Kageboshi, the intelligence agency for the massive integrated enterprise foundation known as Galaxy. His real name is Eishirou, identifying him as a member of the Yabuki clan, a family of ninjas with a unique heritage who have served Galaxy for generations. He is the son of the current clan head, Yabuki Fujinsai, and possesses a deep but conflicted connection to his family's traditions. Unlike his kin who prioritize mission objectives above all else, Eishiro dislikes the clan's callous methods and has chosen to keep his distance, operating instead as a somewhat independent agent within the Kageboshi, which has earned him a reputation as an eccentric even among his underworld colleagues.

His primary motivation is not tied to the glory of the Festa, the school's fighting tournament, a competition he has personally given up on. Instead, he finds purpose in gathering and selling information, as well as maneuvering through the political and criminal undercurrents of Asterisk. He offers his intelligence to Ayato and others, usually for a small price like a meal, but his true loyalties are often ambiguous. As a Kageboshi, he secretly operates under the authority of the student council president, Claudia Enfield, although this relationship is fraught with tension and mutual manipulation.

As the story progresses, his role shifts from a supportive friend to a direct obstacle. When the Yabuki clan attempts to assassinate Claudia, Eishiro is forced to follow orders and confronts Ayato personally. In a bare-handed duel, he uses his clan's ninjutsu techniques to battle his roommate, only to be defeated. He provides Ayato with the information needed to save Claudia as promised, but his betrayal of the clan puts him in significant danger, nearly leading to his elimination by his own father. Following these events, Claudia, who had long been aware of his activities, forcibly reels him in, making him her personal spy. He eventually joins the alliance opposing the main antagonist group, the Golden Bough Alliance, though his loyalties remain permanently colored by his duplicitous nature.

Eishiro is a capable fighter. Even though he does not compete in the Festa, his combat skills are noted to be on par with the elite Page One students of the academy. He is proficient in the ninjutsu of the Yabuki clan, which includes techniques such as creating black fog clouds to disorient enemies, masking his location, and setting traps. He notably fought Ayato to a standstill in close-quarters combat, despite admitting his opponent may have been holding back. As a Genestellar, he possesses the enhanced physical abilities common to his generation. In addition to combat, he is a master of information warfare, capable of hacking, spreading disinformation, and providing practical support such as lock-picking or arranging for covert backup. This combination of social charm, intelligence expertise, and lethal physical ability makes Eishiro Yabuki a uniquely unpredictable and valuable asset in the conflicts of the waterborne city.