Yusuke Ninomiya, known as Yū, is an academic prodigy who uses a wheelchair in his home world. He survived a plane crash that killed his parents and paralyzed his legs. Spending his childhood in a Tokyo hospital, he met an old man who claimed to travel between worlds and wield magic. Later, a flower shop owner adopted him, giving him an adoptive sister, Saki Mishima.

Yū forged a deep childhood bond with Haruto Ichihara (Haru) when he defended Haru from a dog, throwing a stone that redirected the attack onto himself. This bravery cemented their lifelong friendship. In high school, both Yū and Haru developed feelings for classmate Kotona Takashina, though Haru ultimately dated her. The trio spent time together, though inaccessible locations sometimes limited Yū’s participation.

A critical moment came when Kotona called Yū in distress, pursued by an assailant. With Saki’s help, Yū found Kotona just as the attacker stabbed her. He caught her as she fell, and Haru arrived moments later. Their attempt to carry Kotona to safety led to a near-collision with traffic, abruptly transporting Yū and Haru to another world. There, Yū discovered he could walk unassisted.

Arriving in the city of Evermore, they wore unfamiliar clothing and carried compasses instead of phones. They encountered a small pink dog-like creature and sought aid from mercenary Bauer Linden. Noticing Princess Astrid of Evermore’s resemblance to Kotona, they infiltrated the palace and found Astrid unconscious and cursed. Yū removed a dark dagger from her midsection, breaking the curse and awakening her. Though arrested for trespassing, Astrid defended them, securing their release.

Yū grew attached to Astrid and accompanied her to the Lake of Purity to cleanse residual curse energy. During the journey, he spotted the legendary Divine Avnaron bird. After a successful arena battle, the king’s advisor accused Yū and Haru of treason. Realizing danger triggered world travel, Yū convinced Haru to leap into a fire, returning them to Tokyo.

Back home, they learned Kotona had a malignant tumor with a three-month prognosis. Yū deduced a life link between Kotona and Astrid, suspecting Astrid was endangered. When Haru proposed killing Astrid to save Kotona, Yū opposed the plan. The assassin reappeared, transporting all three back to the other world. Yū was captured and forced to fight Haru to prove his loyalty. Their duel transported them back to Tokyo after they nearly fatally wounded each other.

Yū later helped rescue Saki from the assassin, who transformed into a spider-like creature. Returning to the other world, they uncovered a conspiracy: the king’s advisor and the enemy commander were both the crown prince, Sedulus Astrum (Galeroth). Galeroth magically compelled Haru to attack Astrid, but Yū intervened, using newfound magical abilities to fight Galeroth. In the struggle, Galeroth impaled Yū with debris. Haru and Astrid fought Galeroth but faltered until Yū, empowered by the Divine Avnaron and the wizard Mornstar, assisted Haru in delivering the fatal blow.

The resolution revealed Yū and Haru were soulmates, with Yū originating from the other world, not Haru’s. This connection explained why Haru retained memories of Yū while others in their home world forgot him. Yū chose to remain in the other world, entering a relationship with Princess Astrid and becoming Prince of Evermore. Throughout his journey, Yū evolved from a reserved student to a decisive leader and magic wielder, demonstrating tactical intelligence, bravery, and loyalty in protecting those he cared for.