Yuno Gasai is obsessively in love with Yukiteru (Yandere) as one can safely guess from just the simple name of her future diary. Yukiteru’s condition and actions are recorded every ten minutes in her “Yukiteru” diary, it also provides small commentary made by Yuno in some of the entries. Its most obvious weakness is that it does not show Yuno’s future at all—her condition, situation, or surroundings—unless it somehow involves Yuki. Working with Yukiteru’s “Random Diary” diary, however, covers both of their diaries’ individual flaws; as it has been said the two make for a perfect combo.
Smart, beautiful and diligent—those who don’t truly know Gasai Yuno would simply see the model student role she plays while at school. She is secretly obsessed with Yukiteru and would do anything within her power to assist him. She can be quite clingy and is highly fond of stalking him. When obstacles arise that either endangers Yukki’s life or threatens their relationship, her thoughts turn murderous and she lets nothing get in the way of her path. Yuno is also able to quickly analyze and adapt to unexpected situations that may arise, and she puts thoughts into action without hesitation, performing what needs to be done without ever losing her nerves. These traits make her one of the strongest players in Deus’s game, despite having one of the weakest diaries for providing self survival aid. And while Yukiteru is credited for triggering the greatest shift in future occurrences, Yuno is the one who has created the largest influence in those events. Yuno tends to carry around a box cutter with her at all times, though she will make use of other weaponry such as kitchen knives, hatchets, etc, when required.
She is named after Juno (pronounced yu-no in Latin), the Roman goddess of Motherhood and Femininity.
Near the end, when Yuki and company begin to seriously question Yuno’s motives, Aru Akise investigated the 3 nameless corpses in Yuno’s closet. He discovered two were Yuno’s parents, whom she had accidentally starved to death, and that the last one was a perfect DNA match to Yuno herself. While Yuki shrugged this off initially, Aru investigated further and discovered that the Yuno Gasai he knew was actually from a separate dimension that had already won Deus’ game and become god, at the price of Yuki’s life. She then set out to the dimension where the story takes place, killing this worlds Yuno and taking her place by Yuki’s side in her second game. Yuno kills Aru before he can specifically tell Yuki about the situation.
After a final confrontation between herself and Mur Mur against Yuki and Ninth, she heads to a third world where she hopes to try again, trapping Yuki in an alternate reality. Yuki is able to break free and manages to save the third world’s Yuno before first Yuno attempts to kill her. Yuki talks her down, and first Yuno commits suicide by stabbing herself in the abdomen, causing Yuki to become god.