Hajime Mizuki is a third-year student at St. Rudolph Gakuin, where he serves as both a player and the team's de facto manager and strategist. While the official captain is Akazawa, Mizuki holds the true authority within the group, making critical decisions regarding training, formations, and player recruitment. He was born on May 27th, making him a Gemini, and stands 166cm tall with a right-handed all-rounder playing style. Known for his signature high-pitched laugh and a habit of twirling his hair around his finger, Mizuki presents a polite demeanor that masks a deeply calculating and ambitious personality.

Mizuki’s core motivation is to build an elite team capable of winning the national championship. To achieve this, he actively scouts talented players from across the country to transfer to St. Rudolph, bolstering a squad he considered otherwise weak. His strategic mind is his greatest asset; he excels at data tennis, a style focused on meticulously observing opponents to identify their weaknesses and predict their team's lineup and match scores. He uses this intelligence to craft detailed game plans, believing that with perfect data, any opponent can be defeated. This confidence borders on arrogance, and his methods can be ruthless. He taught his teammate Fuji Yuuta the Twist Spin Shot, a highly dangerous technique known to cause severe and lasting injury to a young player's arm, prioritizing victory over his teammate's well-being.

In the story, Mizuki serves as a significant rival, particularly to Seishun Gakuen's data master, Inui Sadaharu. He acts as a foil to Inui; while Inui learns that data has limits, Mizuki arrogantly clings to the belief that his analysis is infallible. His most pivotal relationship is with Yuuta and Yuuta's older brother, the prodigy Fuji Shuusuke. Mizuki exploits Yuuta's resentment towards his brother, manipulating this bitterness to recruit him and further his own goals. This manipulation directly leads to a climactic match where Mizuki faces Fuji Shuusuke. Confident that his complete data on Fuji guarantees victory, Mizuki is instead utterly humiliated. Fuji deliberately allows Mizuki to win five games before revealing his true power and winning seven consecutive games, serving Mizuki a massive slice of humble pie.

Following this decisive defeat, Mizuki undergoes noticeable character development. He becomes less of a jerk to Yuuta and begins training him properly, even proclaiming the younger Fuji as his disciple in a style reminiscent of Inui. This marks a turning point where his ambition begins to temper with a degree of genuine care for his team. His journey continues when both he and Yuuta are invited to the elite U-17 training camp, despite their school not qualifying for the Nationals. There, he is placed in a room with other data tennis players, including Inui and Renji Yanagi, and competes to advance through the camp's rigorous courts, showcasing his abilities on a larger national stage.

As a player, Mizuki is a technically skilled all-rounder with strong mental fortitude and technique, though his physical speed and power are only average. His primary ability is data tennis, which he uses to control the rhythm of a match and systematically dismantle his opponent's game. He is also revealed to have been a child prodigy in table tennis, having once been a candidate for the national team, and he remains exceptionally skilled at the sport. His diverse talents extend off the court as well; he is a polyglot with proficiency in Japanese, English, French, and Spanish, and his hobbies include selecting black tea leaves and keeping a rose observation diary.