Eishin Kuga is a 22-year-old male with black hair, marked physically by prominent burn scars across his chest resulting from severe childhood abuse. After a troubled past, he grew up in a juvenile institution. As an adult, he lives as a problematic delinquent, defined by his refusal to be controlled and his confrontational nature. His formidable fighting skills earned him the moniker "the mad dog." Despite this intimidating reputation, he reveals a teasing, fun-loving side with those close to him.

Kuga is in a same-sex relationship with Kanji Kageyama, a doctor operating a clinic frequented by yakuza members. Their relationship began in the prequel story "Don't Stay Gold." Kageyama possesses a known fetish for scars and scabs, a trait directly relevant given Kuga's extensive scarring. Kuga's background and relationship connect to the main narrative through Kageyama's long-standing friendship with yakuza boss Yashiro, who harbored unrequited feelings for Kageyama during their high school years.

Kuga appears across multiple adaptations. He features in the original manga, including the prequel stories compiled in the first volume. In animation, he appears as a secondary character in the film "Twittering Birds Never Fly: The Clouds Gather" and has a main role in the original animation DVD "Twittering Birds Never Fly: Don't Stay Gold," which adapts his introductory story.