Ibu Shinji is a second-year student at Fudomine Middle School and serves as one of the team's two aces. Born on November 3rd, he stands 165 centimeters tall and weighs 55 kilograms, with AB blood type. His family consists of his parents, two younger sisters, and a younger brother, and his father works as a civil servant. Among his academic subjects, his favorite is physics, while he has less confidence in English due to a dislike of pronunciation.

In terms of personality, Ibu is most famously known for his habit of constantly murmuring and talking to himself under his breath, a trait that frequently annoys his teammates. He rarely shows any change in his facial expression, which has been compared to a ceramic mask, making it nearly impossible for others to read his emotions or thoughts. Despite this stoic exterior, he holds a deep sense of loyalty to his team captain, Kippei Tachibana, and a strong commitment to his teammates. He can be quite critical of things he does not like and is known for his straightforward, sometimes harsh, way of speaking.

Ibu's motivations are closely tied to the revitalization of the Fudomine tennis team. During his first year, he and his peers suffered under a regime of hazing and violence from older students, preventing them from becoming regulars. This period of suffering ended when Kippei Tachibana arrived and reformed the team, an event that explains Ibu's profound respect for his captain and his determination to secure victories for the team. His role in the story is primarily as a formidable singles player, whose talents are recognized by others, with Coach Sumire Ryuzaki even commenting that his natural abilities rival those of Seishun Gakuen's prodigy, Shusuke Fuji.

His key relationships center on his fellow Fudomine members. He frequently partners with Akira Kamio, the other ace of the team, to form a doubles pair when necessary, despite Ibu's preference for singles. He looks up to Captain Tachibana, following his decisions without question and embodying the new, more disciplined spirit of the school. His first major match against the protagonist, Ryoma Echizen, is a significant relationship in the narrative, as Ibu's technique forces his opponent into a difficult position and causes an injury to Ryoma's eye, making it a pivotal early conflict.

Throughout the series, Ibu shows development as a player and a person. In the Kanto Tournament, he and Kamio faced the powerful pair of Genichiro Sanada and Renji Yanagi from Rikkai University附属, experiencing a crushing defeat that highlighted the gap in their skills. During the National Tournament, he demonstrated his resilience by attempting to continue his match against Kintaro Toyama of Shitenhoji despite suffering a sprained arm, showing an emotional outburst unusual for him when he was forced to retire. Later, he receives an invitation to the U-17 training camp, where he is initially placed in the losing group but continues to train and improve with his peers.

Ibu's notable abilities on the court are marked by his high technique and tactical intelligence. His play style is that of a counter-puncher, known for wearing down opponents from the baseline. He possesses an incredible ability to concentrate, which allows him to analyze opponents and execute precise strategies. His signature techniques include a powerful kick serve, which has a sharp, high bounce similar to a twist serve. His most famous ability is the Spot, a technique where he alternates between topspin and slice shots, causing a temporary, one-second paralysis in the opponent's muscles that makes them drop their racket. His precision and feel for the ball are such that he can dribble a tennis ball continuously on the rim of his racket with his eyes closed. Beyond tennis, his interests include listening to music, studying historical data, and his favorite food is pickles.