Yuno Gasai is the yandere to Yukiteru Amano in the anime Future Diary. She's often referred to the "Queen of Yanderes".
Yuno was adopted and for a while, lived an incredibly stable and peaceful life. However, when her mother began torturing her (locking her in a cage, timing the things she does), and her father started coming home later and later, Yuno began breaking down. When she locked both her parents in the cage to give them a taste of how it would feel, she mistakenly forgot to feed them, which caused their deaths. As of this time, it was when Yuno started looking for other emotional support systems, something she desperately needed. She found it through Yukiteru Amano, a fellow classmate, who suggested the two get married when they're older. It was this dream Yuno clung to, if only to keep her sane.
Because of this, whenever Amano is with other people, such as friends or allies, she would go out of her way to drive them away, if Amano leaves her unchecked. She becomes more violent and unhinged when she believes Amano is being threatened, or if someone else is after his heart.
It is later revealed that Yuno had won the 1st reiteration of the survival games, after tricking Amano into suicide. Later, when she wins, she attempts to revive Amano, only to find out that though the body may be revived, the soul cannot. Thus, with Murmur's help, she travels back in time and kills the Second World's Yuno. She then takes the 2nd Yuno's place in the game, if only to ensure Amano survives.
At the end of the first episode, she cradles her face while creepily declaring that she will protect Yuki during the games. This spawned the character base for what is nicknamed "the Yandere Face". Thousands of pictures with different characters were created and are found all over the web. There's even a video showcasing various characters doing the Yandere Pose.