She's a popular girl who is secretly the stalker of Yukiteru Amano. Her future diary is the [Yukiteru] Diary.
She goes to the same school and class as the protagonist, Yukiteru Amano. Contrary to him Yuno is quite popular and a model student at school with perfect grades. Unknown to anyone though she has many secrets. She's in love with Yukiteru and stalks him every day without the boy being aware of it. She's willing to do anything for her love, something which Yukiteru always underestimates in the story.
NOTE: Due to the importance of Yuno's characters this entire section is spoiler. Be warned!
Gasai Yuno is introduced as early as in the first chapter. She appears to know about the Future Diaries and even reveals hers which initially freaks Yukiteru out. He falsely believes she's trying to kill him and takes it for the run. Later when he finds the real killer Yukiteru and Yuno teams up and successfully defeat the man. After the battle they remain as a team to survive Deus's game. Yukiteru's [No difference] Diary which predicts everything happening around Yukiteru except Yukiteru himself and Yuno's [Yukiteru] Diary which displays nothing but Yukiteru's future forms an incredible combination. They force the 9th to retreat with the additional help of the 4th. She and Yukiteru are getting closer until he visits Yuno's house. In a sealed room he finds three corpses and realizes that Yuno's crazy and psychotic. It naturally scares him out and tries to distance himself from her though he didn't break up their alliance.
[to be continued...]
Yuno Gasai despite her very limited Future Diary regarded as the most potent and dangerous player in the game. It is primary thanks to her sharpness and her surprising fighting ability. She's great both at planning and battle. While teaming up with Yukiteru she takes on the role of the attacker and acts as a guardian of Yukiteru while the later checks his Future Diary and deals with the "peaceful" part.
[Yukiteru] Diary: Yuno's Future Diary. It reports on Yukiteru's status in every 10 minutes. With this Yuno can always tell what will happen to Yukiteru and can save her anytime. Although other Future Diaries may confuse the future and the [Yukiteru] Diary's contents change accordinglyg. Her Future Diary is like a cellphone and as with any other Diary users, destroying the Future Diary equals her death.