Tatsuya Asagiri, the main protagonist across all official media, lives in Mitsurugasaki City, Earth's diplomatic hub for relations with the lunar Kingdom of Sphere. Following his father's unclear disappearance and his mother's subsequent death, he resides with his cousin Sayaka Hozumi and his adopted younger sister, Mai Asagiri. He attends St. Catherine's Academy and works part-time at Trattoria Samon, the Italian restaurant next door owned by the Takamizawa family. Tatsuya raises multiple dogs, reflecting his consistent affinity for animal care. His longstanding fascination with lunar culture stems from a childhood encounter with a girl from the moon, later revealed as Princess Feena Fam Earthlight.

Friendly, diligent, and kind, Tatsuya often teases his childhood friend Natsuki Takamizawa. He unconsciously pinches girls' noses as a sign of affection, noted across all media. His initial apprehension about hosting Feena during her homestay evolves into deep loyalty and support for her mission to bridge Earth-Moon relations, honoring her late mother's dream.

In the visual novel and its expanded versions (PS2, PSP, *Moonlight Cradle*), his relationships vary: he pursues romantic routes with Feena, his escort Mia Clementis, Natsuki, Mai, the mysterious lunar being Wreathlit Noel, classmate Midori Tōyama, or the lunar priestess Estel Freesia. Key developments include:
- In Feena's route, he trains in kendo to prove dedication despite societal opposition, solidifying their bond and catalyzing Earth-Moon reconciliation;
- In Mai's route, he confronts their shared secret of non-blood relation, culminating in discarding symbolic ribbons to transition from siblings to lovers;
- In Natsuki's route, he navigates anxieties over her departure for veterinary studies, reinforcing a childhood promise;
- In Estel's route, he patiently addresses her anti-Earth prejudices rooted in her orphaned background, aided by discovering her love for dogs.

The anime adaptation focuses exclusively on his relationship with Feena, depicting their childhood connection, her homestay, and their eventual marriage formalizing the Earth-Moon alliance. *Moonlight Cradle* explores aftermath scenarios, such as maintaining relationships post-confession and facing new diplomatic challenges. His actions consistently emphasize bridging cultural divides through personal bonds and diplomatic efforts, underpinned by his knowledge of lunar society and commitment to peaceful coexistence.