Seigi Nakata is a second-year business student at Kasaba University in Tokyo, living alone in Takadanobaba. His early life carries strong familial influences, particularly from his grandmother Hatsu Kanou, who instilled a fierce sense of justice reflected in his name ("Seigi" meaning "justice"). Childhood classmates mocked him for helping others, yet his grandmother's affirmation solidified this core value. His mother, Hiromi Nakata, worked extensively, prompting Seigi to develop self-reliance in domestic tasks like cooking, with pudding becoming his specialty. During elementary school, he practiced karate to manage anger stemming from his abusive biological father, eventually earning a black belt. This training provided self-defense skills and an emotional outlet, though he discontinued formal classes upon entering university. He also learned about his grandmother's past as a pickpocket, a revelation that complicated yet did not diminish his admiration.

Financial constraints led Seigi to take a night security job at a Shibuya television station, resulting in exhaustion and irregular sleep. His life shifted during an encounter near Yoyogi Park, where he intervened to rescue Richard Ranasinghe de Vulpian from harassers. This meeting prompted Seigi to seek Richard's expertise in appraising a ring inherited from his grandmother, leading to part-time employment at Richard's Ginza jewelry store, Jewelry Étranger. This role redirected his career aspirations from civil service toward gemology and customer assistance.

Psychologically, Seigi exhibits extroversion and friendliness but battles PTSD from paternal abuse, manifesting in anger management challenges and fears of perpetuating violence. He undergoes therapy with mixed success, and suicidal tendencies persist. This trauma fuels avoidance of romantic relationships due to anxieties about becoming an abuser. Early interactions sometimes reveal unintentional insensitivity or microaggressions, though he gradually cultivates greater emotional awareness through experiences at the jewelry store.

His development includes refining his appearance from functional attire to formal business wear suitable for his jewelry work. He pursues language studies under Richard's guidance, achieving fluency in English and Sinhala, with additional knowledge of French, Spanish, and Mandarin. This facilitates his relocation to Sri Lanka as Richard's apprentice, where he adopts two mixed-breed dogs, Jiro and Saburo, and starts an English-language blog under the pseudonym "Iggy" documenting cultural experiences and dessert recipes.

Key relationships define his journey. His bond with Richard evolves from employer-employee to mutually declared "partners," involving deep emotional reliance and declarations of devotion. Their dynamic includes physical expressions like near-kisses, forehead kisses, and finger kisses, with the relationship's nature intentionally ambiguous. Classmate Shouko Tanimoto represents an initial romantic interest, though her asexual and aromantic orientation leads to a lasting platonic friendship where she provides mineralogy expertise. Familial reconciliation occurs with his mother, transitioning from childhood conflict to mutual understanding, while his abusive biological father remains a source of unresolved trauma.

Professionally, Seigi transitions from part-time shop assistant to skilled jeweler's apprentice in Sri Lanka. His journey integrates gemology expertise with emotional growth, confronting his past and defining his identity beyond societal expectations. The narrative avoids conclusive romantic categorization, focusing instead on his evolving self-acceptance and commitment to supporting others.