Eiichi Misumi is a second-year student at Otobuki High School and a member of the school's Game Club. He is a classmate of Karen Tendou, who invites him to join the club at the beginning of the series. Physically, he is a young man of average height with short black hair and gray eyes.

Due to a mysterious incident in his past, Eiichi has no memory of his life before a certain point, stating that he woke up in an arcade with no identification or memories. This amnesia profoundly shapes his personality and motivations. He reveals that playing puzzle games, particularly block puzzles, gives him a link to his forgotten past, as playing them occasionally triggers fragmented memories. Consequently, puzzle games are his specialty, though he demonstrates a remarkable ability to adapt to and learn new games by observing how others play.

Despite his mysterious background, Eiichi is consistently portrayed as a kindhearted, friendly, and sociable individual. Upon first meeting the protagonist Keita Amano, he openly expresses a desire to be friends, citing a lack of close associates. Even after Keita declines to join the Game Club, Eiichi remains friendly, spending time with him and even seeking his game recommendations. This supportive nature extends to his other club members. He is notably perceptive when it comes to the emotions of those around him, quickly recognizing that Karen has romantic feelings for Keita. He offers her advice on how to reject other boys' confessions more gently, finding her typical manner too cold and painful for the boys involved. While he demonstrates considerable awareness of the romantic dynamics between his friends, Eiichi himself appears to be quite oblivious when it comes to matters of the heart involving himself. For instance, he fails to realize that his adoptive sister, Riki, has developed romantic feelings for him as a result of his past kindness.

The details of Eiichi’s past are a key part of his character. He was found in a city wearing a military-like uniform with the letters E.G.G. on an armband and a small handgun in his pocket. At some point, he came across a young girl named Riki who was being confronted by men in black suits. He helped her escape, and as a gesture of gratitude, her wealthy family adopted him, giving him the name Eiichi Misumi. However, his life did not become ordinary. He continues to be contacted by a mysterious organization that was after his adoptive sister and occasionally has flashes of being part of unknown experiments performed by these same people. The series later shows a glimpse of his secretive past, revealing that he had escaped from some underground organizations and eventually encounters Riki again in the finals of a game tournament.

Throughout the series, Eiichi’s role is that of a supportive friend and a somewhat grounded observer amidst the romantic misunderstandings and comedic miscommunications that drive the story. He interacts frequently with Keita, Karen, and other main characters, often acting as a catalyst for them to realize or confront their own feelings. While he initially struggles to make friends in his first year of high school, his involvement in the Game Club and his open, friendly demeanor allow him to build meaningful friendships by his second year. His character development is tied not to a dramatic personality shift, but to the gradual revelation of his mysterious origins and his navigation of ordinary high school life, which stands in stark contrast to the extraordinary and mysterious circumstances of his early life.