Eiri Yuki, born Eiri Uesugi, is a renowned romance novelist using a professional pen name. His family holds a religious background; his father is a monk managing a temple in Kyoto. He has an older sister, Mika Seguchi, married to Tohma Seguchi, and a younger brother, Tatsuha Uesugi, training as a monk. His mother died before the series' events.

At sixteen, Eiri endured severe trauma in New York City when his mentor, Yuki Kitazawa, arranged an attempted sexual assault by criminals for money. During the attack, Eiri grabbed a gun and killed Kitazawa and the assailants. This triggered post-traumatic stress disorder, causing memory repression and a drastic personality shift from cheerful and trusting to emotionally detached, sarcastic, and caustic. He blamed his former trusting nature for the betrayal, fostering deep-seated commitment issues and reluctance to form emotional bonds.

Eiri first encountered Shuichi Shindo when lyrics from the aspiring musician blew towards him in a park. His harsh criticism of Shuichi's work initiated a tumultuous relationship characterized by avoidance and emotional distance. Despite his cold exterior, Eiri gradually developed genuine feelings for Shuichi, struggling to express them openly. His initial engagement to Ayaka Usami dissolved as his bond with Shuichi deepened.

Later, Eiri and Shuichi temporarily cared for Riku, the orphaned son of his deceased mentor Kitazawa. During this time, Eiri suffered a car accident while saving Riku from an oncoming truck, resulting in temporary blindness. It was later revealed he faked the blindness to regain Shuichi's attention following a misunderstanding involving Ryuichi Sakuma, demonstrating unexpected emotional vulnerability and manipulation.

Distinctive personality traits include sleeping on his stomach, despising celery due to a childhood incident with his sister, and being left-handed. He enjoys strawberry cakes, chocolate pie, and Budweiser beer, while smoking Marlboro cigarettes. He shows fondness for a neighborhood cat named "Mr. Meow" and has ticklish ears. Professionally, his romantic writing contrasts his personal reticence; a significant example is composing "Shining Collection," lyrics expressing love for Shuichi, performed publicly by Nittle Grasper.

Over time, Eiri underwent therapy for his trauma, and his doctor eventually declared him cured of the PTSD from the Kitazawa incident. His volatile relationship with Shuichi prompted gradual emotional opening, including moments of public acknowledgment. Despite ongoing trust and communication challenges, Eiri demonstrated increased protective instincts, notably when threatening Shuichi's kidnappers.