She has been stalking Kantarou for several months. She confessed her love to him on multiple ocassions. One night, when she was spying on him, she saw him kissing his girlfriend, Sanae.
She's mentally unstable and violently obsessed with Kantarou. When she saw him kiss his girlfriend, she was practically in shock, trying to process the event. In a defensive moment, she believed a story that she was dating Kantarou and that he was being stalked and harassed by Sanae. She attempted to "free" him by trying to kill Sanae. However, Kantarou got in front of Sanae and suffers from amnesia from Nagisa's attack. Nagisa takes advantage of his state and tells him that they were together. While she spends a lot of time with him, his real girlfriend is taking things slow and hoping that he would remember her. On the fourth day of his awakening, she corners Sanae in his house and kills her. Kantarou shows up afterwards and Nagisa happily admits her crime. Kantarou remembers what happened before he lost his memories and he calls out Nagisa. She tearfully admits that everything she said is true, unable to accept the truth. For the True End, she attempts to kill both of them so that they can be together in the afterlife. Before she dies, she makes a claim that when she goes to Heaven, she will become God and make everyone in the world happy.