Takuya Arima, a third-year Sakaimachi Academy student with blue hair and black eyes, lost his biological mother Keiko Arima young, retaining only fragmented memories. His historian father, Kodai Arima, was declared dead after vanishing in a rockfall during research. Takuya then lived with his stepmother, Ayumi Arima, their relationship strained by her grief and work pressures.

He inherited his father's Reflector Device, enabling travel across parallel worlds and timelines. Initially seeking answers about Kodai's disappearance, Takuya uncovered larger interdimensional conflicts. Utilizing the device's Automatic Diverge Mapping System (A.D.M.S.), he navigated branching storylines, collected power-source jewels, revisited timelines to alter outcomes, and discovered Kodai survived in another dimension, developing theories on parallel histories.

Academically capable but often unmotivated, Takuya possessed a lecherous streak marked by inappropriate remarks toward women. Yet deeper traits emerged: perceptiveness, bravery, and a strong moral compass. He protected allies like Kanna Hatano, whose life depended on a psychite amulet, and intervened across timelines to prevent Ayumi's suicide. His determination led him to challenge figures like the interdimensional impostor posing as Principal Ryuuzouji, who sought the Reflector Device for destruction.

Transported to the alternate dimension Dela Grante, Takuya encountered the mute girl Sayless. He fathered a child, Yu-No, with her. Following Sayless's death and Yu-No's capture, he allied with resistance leader Amanda to storm the imperial capital. There, he found Ayumi, transported earlier, now Dela Grante's Divine Emperor attempting to save both worlds by sacrificing Yu-No to a world-sustaining AI. This failed, causing Dela Grante to collide with prehistoric Earth.

In the aftermath, Takuya learned Kanna was his daughter from another timeline. He fulfilled a promise by giving her an unbreakable psychite. His final act involved residing eternally with Yu-No and Sayless in a non-temporal rift to preserve reality, witnessing the birth of a new universe symbolized by a sprout.

Takuya's relationships were complex: romantic tension with Ayumi complicated by their step-relationship; a past affair with teacher Mitsuki Ichijou; belligerent chemistry with classmate Mio Shimazu; protective instincts toward Kanna, later recognized as his daughter; and a defining bond with Sayless and Yu-No in Dela Grante that fueled his resolve against fate.

Situationally proficient in combat, he outwitted adversaries like Hojo and defeated monsters, though he also suffered defeats. The Reflector Device's mechanics underscored his reliance on revisiting timelines to achieve optimal outcomes, though fixed events like Mitsuki Ichijou's death remained unalterable.