Izayoi Sakamaki emerged from a rootless existence in foster care, shuffled between orphanages labeled difficult. He eventually initiated a high-stakes game challenging others to find him within a time limit, seeking someone of equal skill. Canaria discovered him moments before time expired and proposed a wager: his loss meant becoming her son. Accepting, he traveled with her for two years, absorbing extensive knowledge of mythology and scientific concepts. An incident at Iguza Falls saw him destroy half a dam out of disappointment over a missing legendary demon, forcing their escape. Canaria later founded an orphanage before falling terminally ill. Honoring her final wish, he enrolled in high school. After her death, he received both a mysterious letter and a summons concerning her will. The will contained a riddle linked to disappearances at the foster home, leading to a confrontation with the death god Baron La Croix. Surviving revealed Canaria's posthumous challenge: open the letter to enter a world of perpetual conflict or remain in his carefree life. He chose the former, seeking greater challenges.

Transported to Little Garden alongside Asuka Kudō and Yō Kasukabe, he immediately demonstrated sharp perception by detecting Black Rabbit's presence. His first act involved challenging her to a trial Gift Game, which he won effortlessly. Instead of accompanying the group, he ventured to the world's edge to confront a water god at Tritonis Falls, defeating it and securing a water tree sapling vital for restoring the barren territory of the No-Names community. He deduced Black Rabbit's true motive for their summoning—No-Names' decline after losing their name, banner, and members to a Demon Lord—and pledged assistance, driven by the prospect of facing powerful adversaries and restoring the community's standing.

His capabilities center on "Code:Unknown," an ability defying appraisal by the Fragment of Laplace and enabling the negation or destruction of other supernatural Gifts. He possesses immense physical strength, speed, durability, sensory acuity, and limited light manipulation. Later revelations indicate potential to destroy stars, though unlocking this requires specific triggers. His combat prowess allows him to subdue gods and mythical beings, while his intellect solves complex Gift Game riddles, like the Pied Piper of Hamelin scenario. He maintains extensive knowledge of global mythologies and scientific principles, often applying this strategically. Following the destruction of his headphones and school uniform during the Underwood arc, he adopted new attire: a yellow shirt, red zip vest, black jacket, gray pants, and brown boots, later incorporating miner's equipment for specific challenges. He later gained additional abilities like [Last Future of Embryo], Aurora Pillar, and Leo's Sun Authority.

His personality blends intellectual curiosity with a hedonistic pursuit of excitement. He self-describes as vulgar, savage, and unpredictably drawn to conflict, frequently engaging in perverted banter targeting Black Rabbit's physique or virginity. He participates only in endeavors he deems interesting, willingly accepting handicaps to enhance challenge. Despite a flippant exterior, he exhibits high intelligence, solving intricate puzzles and demonstrating broad expertise. He adheres to a personal moral code: "Trample both the strong and the weak," showing no inherent mercy to any opponent, god, ally, or Demon Lord. However, he values fairness and despises those exploiting the vulnerable, especially children. A sore loser, he obsessively practices skills after defeats, as seen when relentlessly honing cooking after losing to Yō. Loyalty underpins his actions; he rejects offers from stronger communities to aid No-Names' restoration and flies into unrestrained rage when comrades like Black Rabbit suffer severe harm. His core motivation remains "to have fun," yet he increasingly shoulders burdens alone, believing his power isolates him from dependable allies.

Development reveals a deepening commitment to No-Names beyond initial thrill-seeking. He actively strategizes to rebuild their reputation, using leader Jin Russel's name as a rallying point after the Fores Garo defeat. He mentors Jin, promising to retrieve their enslaved former member, Leticia Draculair. His protective instincts extend to community children, confronting spies and offering vengeance for murdered hostages. Wardrobe changes symbolize growth from a detached troublemaker to an invested protector. Confrontations with entities like His Highness during the Alliance Flag arc force him to acknowledge emotional stakes beyond personal amusement, leading to uncontrolled fury when allies are endangered. In later arcs, such as Last Embryo, he faces escalating threats and responsibilities, further refining his abilities and leadership while grappling with the isolation his power imposes.