Issei Ryuudou is a student at Homurahara Academy and serves as the diligent and serious student council president. He is in the same class as his close friend, Shirou Emiya, having first become acquainted with Shirou during their first year of high school. Issei is the son of the head priest of the Ryuudou Temple, a traditional Buddhist temple in Fuyuki City, where he lives. His home life at the temple, which follows a strict and frugal regimen with little meat in its meals, has shaped much of his disciplined character. He plans to inherit the temple and become a monk after graduating, a path he takes seriously despite admitting his own lack of spiritual sensitivity.

In terms of personality, Issei is known for being straight-laced, serious, and honest, with little interest in leisure or modern trends. While generally polite and courteous, he transforms into a notoriously strict and even fearsome figure when performing his duties as student council president. He is highly principled and does not break school rules, such as running in the hallways, no matter the circumstance. As an administrator, he is passionate about reforming the school, particularly the unfair distribution of club activity budgets, a crusade that has earned him both respect and opposition.

One of Issei’s most defining relationships is his long-standing rivalry with his classmate, Rin Tohsaka. Their adversarial history dates back to middle school, where he served as student council president and she as his vice-president. Their personalities clashed severely during these two years, leading to a mutual distrust that continues into high school, where he frequently refers to Rin as a fox or a demon. This difficult relationship is often cited as a primary reason for his general awkwardness or discomfort around women, although this does not apply to all women. For instance, he instantly takes a liking to Saber, explaining that he can intuitively tell she is a good person.

Conversely, his friendships with men are strong and uncomplicated. His bond with Shirou Emiya is a central part of his everyday life. He relies on Shirou for his skill in repairing broken school equipment, a talent that helps the student council save money, and the two often share lunch together in the student council office. Issei also greatly admires the substitute teacher, Souichirou Kuzuki, whom he trusts and respects enough to call "brother," often practicing martial arts kata with him on holidays.

Issei’s role in the story is that of a normal person who is completely unaware of the secret magical world and the Holy Grail War. However, his connection to the war is that his home, the Ryuudou Temple, is seized by the Servant Caster to be used as her base of operations. Because he lives there, he is caught in the periphery of the conflict. In the Fate route, he visits Shirou's house to check on him and ends up interacting with Saber, accepting her cover story without suspicion. In other scenarios, he and the other temple residents are often incapacitated by magical forces, such as being rendered unconscious by the corrupted Grail or attacked by other Servants, highlighting the danger that the supernatural war poses to ordinary civilians.

Despite being a normal human with no magical abilities, Issei possesses several notable non-magical talents. He holds a black belt in karate and is known for his physical stamina, which he demonstrates by walking two hours each day to and from school from the distant temple. He is also skilled in analytical thinking, with hobbies that include strategy games like Go and Shogi, as well as crossword puzzles. These traits, combined with his charismatic leadership and sharp observational skills, allow him to be an effective and respected student council president, even earning him the admiration of the student body.