Yuuki serves as the central protagonist of Princess Connect! Re:Dive. His childhood included forming strong bonds, acting as an older brother figure to Rino Inosaki and developing a particularly close friendship with Saren Sasaki. He also helped a lost Hiyori Harusaki find her mother and befriended Rei at summer camp, though neither recognized each other later. He unknowingly assisted school idol Nozomi Sakurai by hiding her from fans.

In the original storyline, Yuuki was forcibly logged into the virtual world "Legends of Astrum" after Akira Mosakuji attached a MiMi device to him in a park. Guided by navigation fairy Fio, he discovered an ability to empower allies during battles, enabling coordinated Union Bursts. He displayed sarcasm, dry humor, and strategic thinking, often devising plans for his guild Twinkle Wish. He formed strong bonds with them, promising to help reach the Tower of Sol's summit. His personality blended selflessness—protecting others at personal risk—with occasional blunt remarks.

Following events involving Mana and Minerva at the Tower of Sol, Yuuki suffered severe amnesia upon arriving in Landosol. Found by the elf Kokkoro—assigned as his guide by Ameth—he retained only his name. This memory loss erased basic knowledge, leaving him childlike; he attempted to eat currency, frequently got lost, and spoke minimally or in broken phrases. He later encountered the perpetually hungry swordswoman Pecorine and the cat-eared assassin Karyl, forming the Gourmet Guild. Despite his vulnerability and initial combat uselessness beyond his empowering "Princess Knight" ability, his core kindness persisted. He defended Karyl after learning of her assassination mission and shielded others like Hatsune from attacks.

Fragments of Yuuki's past resurface. During the "Dangerous Vacation" event, he demonstrated regained strategic prowess coordinating allies to defeat a giant squid and rescue Kokkoro. Pecorine noted his increased assertiveness, though his primary amnesiac state remained. He later experienced a second, more severe amnesia reducing his mental state to infancy, recovering shortly after. Encounters with former Twinkle Wish members triggered depressive episodes; he blamed himself for failing to protect them after vivid memories surfaced. This fueled recklessness, like overusing his Princess Knight power to save Kiiri despite physical deterioration. A pivotal flashback revealed he sacrificed his bonds and memories in a previous timeline loop to restart and save his friends.

Yuuki consistently exhibits several traits. His Princess Knight ability amplifies allies' powers, enabling transformations like Princess Forms or Magical Girl states for characters such as Kasumi and Shiori. This fulfills his expressed wish to become a Magical Girl himself despite gender restrictions, becoming a running gag involving involuntary transformations. He adapts Pecorine's extreme omnivory, eating insects raw. Running gags include frequent animal bites and comedic injuries. He maintains a "Consummate Liar" tendency, often falsely claiming loneliness to connect with isolated individuals like Aoi. Despite often seeming academically challenged post-amnesia and needing remedial math help, he displays cleverness tutoring Suzuna or holding multiple jobs. He attracts romantic interest but remains oblivious.

Physically, Yuuki has unkempt black hair, teal eyes, and an average build. His original in-game attire featured a black-and-teal outfit with a blue cape, shoulder armor, and sword sheath; the anime removed the shoulder plate and leg armor. In real-world segments, he wears Tsubakigaoka High's male uniform.