Kogorō Katsura is a young man from the Chōshū domain who comes from a wealthy family and was rigorously trained in the fundamentals of music from early childhood. He is a disciple of the scholar and revolutionary Yoshida Shōin and is the senior of Shinsaku Takasugi, with whom he shares a childhood friendship. Kogorō is also deeply learned in Dutch studies, which he applies to inventing practical devices such as small amplifiers and speakers.

In the story, Kogorō is one of the loyal disciples who, while searching for their lost master Shōin, encounter the enthusiastic rocker Ryōma Sakamoto. Together with Ryōma and Shinsaku, he forms a band that uses rock music to resist the oppressive Tokugawa shogunate, which controls the populace through brainwashing Heaven's Songs. Kogorō serves as the band's drummer and percussionist. He is one of only five individuals who possess a Peace Soul, a special quality that allows him to channel genuine rock energy.

Kogorō's personality is calm, soft-spoken, and rational, making him easy to approach. He is wise and knowledgeable, often displaying patience and kindness toward others. He can become stern and firm when the situation calls for it, especially when his friends are in danger. Despite his gentle demeanor, he is not afraid to stand up for what is right. He is extremely serious by nature, especially in contrast to the more reckless Shinsaku, and he tends to analyze situations carefully before acting. His rationality makes him the group's de facto straight man, and Ryōma affectionately calls him Sensei because of his mature and learned attitude.

Kogorō has a deep enthusiasm for music and is a fanatical collector of music scores, which he treasures greatly. He is also a gifted inventor, creating items such as portable amplifiers that aid the band's performances. On a personal level, he struggles with household chores and has low physical stamina. He has a pronounced shyness about being naked and avoids public baths. He becomes intoxicated very easily and tends to fall asleep quickly after drinking. His favorite food is acorns, while he dislikes black-colored foods.

Kogorō's primary motivation is to support his friends and honor his master's ideals by using rock to awaken people and overthrow the shogunate's tyranny. He cares deeply for Shinsaku's wellbeing and often acts as a cautionary voice, though he is invariably pulled into trouble by the more impulsive members of the group. Through his musical and inventive contributions, he plays a vital role in the band's journey, gradually gaining confidence and strengthening his bonds with his fellow rockers.