During Elementary School, Amamiya was like a mediator between Ageha and Asaga, and she was always smiling and cheerful. She was bright, intelligent, and extremely popular. As she grew up, she slowly began to feel disgust with the world she lived in, and to escape, she took the Psyren Survey. Upon her return from her first visit, her personality shifted even greater to a darker view of the world.
Even though her nickname is "The Ice Princess" and she has a cold heart towards others to match, she can still find enjoyable things in the world, like Matsuri Sensei's music and, later on, Ageha. When Ageha goes to Psyren and saves her, she becomes more attached to him, although she can still live up to her "Ice Princess" name. When she is worn out or extremely angry, her eyes grow wide and blank and she says cruel words that terrify others.
Thanks to Usui's Delete Spider, her memories were tampered with slightly and her cold-hearted personality had become slightly stronger, though it has always been present since her first visits to Psyren. Yandere Amamiya, named Abyss, has a vicious personality and gets a rush from from violence and bloodshed. If it wasn't for Ageha, she would murder thousands. Both Tsundere (calm & normal) and Yandere (wild & mean) Amamiya like Ageha. However, Abyss likes the "strong and dangerous" side of Ageha. She will often refer to him as her "beloved" and bluntly say she loves him. However, if Ageha says the wrong thing, she will attempt to kill him.