Issei Tsubaki is a supporting character in Full Metal Panic? Fumoffu, best known as the head of the karate club at Jindai High School. He is a young man of notable physical strength and possesses a high level of skill in martial arts, having dedicated himself to rigorous training in karate. However, his most defining personal trait is his extreme nearsightedness. Without his glasses, his vision is severely impaired, which becomes the source of numerous comedic incidents throughout the series. Despite this practical need, he often refuses to wear his glasses in public, as he is deeply self-conscious about their large and unfashionable appearance. He prefers to maintain a cool and aloof demeanor, but this prideful exterior consistently leads to trouble.

Tsubaki's primary motivation and the driving force of his role in the story is his intense and unyielding rivalry with the protagonist, Sousuke Sagara. His pride was deeply wounded after suffering a defeat at Sousuke's hands, and he cannot accept this loss. Consequently, he is constantly challenging Sousuke to rematches in a determined effort to restore his honor and prove his superior fighting ability. Sousuke, often oblivious or simply avoiding unnecessary conflict, typically evades these challenges, whether intentionally or not, which only adds to Tsubaki's frustration. Adding another layer to his character is his significant romantic interest in the female lead, Kaname Chidori. He developed a crush on her after an incident where he saved her from a group of thugs, and he believes that defeating Sousuke is the key to winning her heart. His feelings for her are sincere, but his inability to see clearly without his glasses often complicates his interactions, as he cannot even appreciate her beauty unless she is very close to him.

Despite his strong pride, Tsubaki is generally portrayed as a good sport who takes failure surprisingly well, and he is undyingly loyal to his three devoted disciples, who follow him in his training and his various schemes. His key relationships in the series are almost entirely defined by his rivalry with Sousuke and his unrequited love for Kaname. In a notable comedic twist stemming from his poor eyesight and nervousness, he once accidentally pointed at Sousuke while attempting to confess his love to Kaname, leading to a brief but humorous misunderstanding about his romantic intentions. He also develops a connection with a character named Mizuki; in the final episode of Fumoffu, she falls in love with him after he saves her from a dangerous fall.

In terms of development, Tsubaki remains a largely static character, defined by his recurring and futile attempts to defeat Sousuke. His abilities are considerable; he is an exceptionally well-trained karateka who has achieved a level of power that exceeds that of a normal human. He seems to know a wide array of martial arts techniques and possesses a strong fighting spirit. However, his effectiveness is consistently hampered by a combination of his pride, his easily distracted nature, and his refusal to wear his glasses, which ensures his plans almost always end in failure. His role in the story is primarily that of a comedic obstacle and a rival, providing a recurring source of humor through his earnest but doomed challenges against Sousuke's far more pragmatic and unpredictable brand of combat.