Yuno Gasai appears as a model teenage student, excelling academically and socially. She possesses striking pink eyes and long pink hair, usually styled in pigtails with two front locks secured by red bows. Her typical attire is a blue school uniform featuring a ribboned shirt, skirt, and high socks, though official media depicts variations.

Adopted by Ushio and Saika Gasai during infancy or childhood, Yuno experienced initial happiness. Severe financial strain later triggered brutal parental abuse. Her adoptive mother inflicted psychological torment and physical violence, confining Yuno in a cage, forcing her to consume inedible substances, and starving her. Her father witnessed this neglectfully without meaningful intervention. Approximately at age twelve, Yuno reversed the dynamic, imprisoning her parents in their own cage. Their deaths resulted from starvation due to her unconscious neglect. She preserved their corpses within her home, engaging in delusional conversations as if they lived, and later transported their severed heads.

Psychologically dissociative with a fractured personality, Yuno presents a facade of sweet femininity and shyness, particularly around her romantic obsession, Yukiteru Amano. This masks a ruthless, calculating, and violently protective core. Her instability, rooted in childhood trauma and isolation after her parents' deaths, manifests as yandere traits: extreme possessiveness, willingness to manipulate or kill perceived rivals or threats to Yukiteru, and intense separation anxiety. A purposeless existence ended during a school detention encounter with Yukiteru. Learning of his dream to stargaze with his divorced parents, she proposed becoming his "bride" after he jokingly agreed to marry her when older. She interpreted this promise literally, dedicating her life to relentless stalking and surveillance via her cell phone diary.

Yuno wields the "Yukiteru Diary" (Diary of Future Love), a Future Diary predicting Yukiteru's actions in 10-minute increments. While limited alone, it synergizes powerfully with his Random Diary for comprehensive threat anticipation. Beyond this, she demonstrates exceptional strategic intelligence, rapidly formulating complex plans, identifying enemy weaknesses, and setting intricate traps. She exhibits proficiency with diverse weaponry including knives, axes, and firearms, alongside poison/drug knowledge, infiltration skills, and surprising physical strength enabling her to overpower restraints or adversaries despite her stature. Winning the first Survival Game granted her divine powers as Deus Ex Machina's successor, including telekinetic control over massive structures.

Her narrative spans multiple timelines. In the First World, she won the Survival Game with Yukiteru but poisoned him during a double-suicide pact, spitting out her own pills. Unable to resurrect him with godhood, she time-leaped to the Second World, murdered that timeline's Yuno, and replaced her to re-enter the game. When Yukiteru discovered her parents' corpses and her origin, her manipulation intensified, culminating in his victory after she surrendered by suicide. She time-leaped again to the Third World intending to repeat the cycle but ultimately could not kill Yukiteru. After trapping him in an illusory utopia, she suicided when he rejected her methods, enabling his godhood.

In the Third World, Yuno experienced stability with loving parents and no Survival Game. The OVA "Redial" depicts her receiving memories of her First World self via Murumuru, sensing "something missing." Chosen as the Third World Deus' successor, she used divine powers to reunite with Yukiteru in the Second World, fulfilling their stargazing promise.