Yuuji Sakai began as an ordinary high school student whose life transformed when he discovered he had died and become a Torch, a fading remnant of a human with nearly depleted Power of Existence. His uniqueness as a Mystes stemmed from containing the Treasure Tool Midnight Lost Child, which replenished his Power of Existence every night at midnight. This granted him indefinite life barring total exhaustion before restoration, fostering a fatalism tempered by optimism where he sought meaning through self-sacrifice as proof of existence.

He combined inherent kindness and sociability with a sharp, analytical intellect. Prioritizing others' safety, often at his own expense, he maintained calm strategy during conflicts, earning allies' respect among Flame Hazes. However, he remained unaware of romantic feelings from Shana and Kazumi Yoshida, causing unintentional emotional strain. This obliviousness partly originated from his desire to preserve his ordinary life and avoid disrupting the status quo.

Trained by Shana and later Margery Daw, he developed combat skills and learned to manipulate his Power of Existence, eventually casting Unrestricted Spells. His abilities included wielding the Treasure Tool Blutsauger and generating distinctive silver flames, an omen of the Snake of the Festival's influence. His tactical ingenuity proved valuable against Crimson Denizens.

A pivotal change occurred when he merged with the Snake of the Festival, a Crimson God seeking to end conflict by creating the paradise world Xanadu. This fusion amplified Yuuji's latent traits: his fear of powerlessness shifted into ruthlessness and manipulation. He coldly exploited allies like Kazumi, traumatized Shana by stripping her powers, and prioritized his goals over personal bonds, showing indifference to the suffering caused.

His family background included a stillborn twin brother sharing his name. His parents, Chigusa and Kantaro Sakai, remained largely unaware of his supernatural struggles, though Chigusa inadvertently guided Shana on human emotions, indirectly affecting Yuuji's relationships.

In the climax, Shana confronted Yuuji within Xanadu, prompting him to realize his failure to trust her. After acknowledging her feelings in a kiss, Lamia's spell restored his humanity. He then used another spell to restore the consumed humans of Misaki City before departing for Xanadu with Shana to foster coexistence between humans and Denizens.