Daisuke Nikaido is a third-year student at Meisei Gakuen's middle school and a member of its baseball club. He wears the number one jersey as the team's ace pitcher, though his position is due more to external circumstances than innate talent. His father is the president of Nikaido Electric Industries, a major company that donates generously to Meisei Gakuen, and also serves as the chairman of the school's baseball club alumni association.

From a purely physical standpoint, Nikaido is not an exceptionally gifted pitcher. His pitches are considered ordinary, and his stamina is noticeably weak. The evaluations from other team members are generally low, and it is widely understood that he holds the ace position due to favoritism from the coach and team captain, who are influenced by his father's status. Despite these limitations, he possesses a genuine desire to win and shows emotion on the mound. He listens to the guidance of his catcher and can display a fiery attitude when the situation demands it. He is a more complex figure than his reputation as a privileged player initially suggests.

His motivations are deeply intertwined with a serious health condition. During his first year of junior high, Nikaido was diagnosed with a heart condition. Determined to pitch, he continued to play without informing his teammates, with only his family, doctor, and the team manager knowing the truth. He desperately wanted to fulfill his role as the ace, and his final season was deeply significant, as he did not expect to survive the major surgery that awaited him. His father, believing his son would pull through, supported this decision to allow him to play. Following a successful surgery, Nikaido was left with a second chance at life. He decided not to return to baseball for his high school career, feeling no regret for having kept the pitcher's mound for himself during his time.

In the story, Nikaido serves as an obstacle for the series' main protagonists, the Tachibana brothers, who are more talented but sidelined in favor of him. However, he is not portrayed as an antagonist. After his surgery and retirement from the sport, he transitions into a supportive role. He is seen attending games to cheer for Meisei and even steps in to protect a younger student from trouble, feeling a sense of responsibility for his past actions. His relationship with the Tachibana brothers evolves into one of mutual understanding, with no lasting hard feelings on either side. His personal development involves a humbling journey from a privileged but ailing ace to a spectator with a new lease on life, even as he faces the subsequent loss of his father. His notable ability is not his pitching skill, but his resilience and the force of will that allowed him to compete under the shadow of a life-threatening illness.