Kuruta Yujo lived a strange life. She was born into a very rich family. She got everything she could dream of, her family and two older sisters were loving, and she was so incredibly happy. However, she also became needy. She had to constantly have someone close to her, or she'd start crying or throw a tantrum. For years, she permanently had one of her family members near her, which to some extent made her a hassle. Luckily for her, however, her family didn't mind.

Of course, this couldn't last forever. When Kuruta was 6, it all went wrong At that age, her family was well aware that Kuruta couldn't stand being alone so they made sure someone was with her at all times. At one moment though, her oldest sister Touka, 16 years old at that time, was watching over her. She was alone in the room with Kuruta, with her parents and other sister somewhere else in the house. As she stood up to get a drink, Kuruta screamed for her not to go. Touka tried to calm the child down... but Kuruta was having none of it. She grabbed her sister's wrist and held on with surprising strength. When the older girl tried to get herself free, Kuruta bit down in her wrist, drawing a lot of blood. Touka screamed loudly and quickly pulled her hand back, trying to run away, but Kuruta didn't let her. She jumped up onto her sisters back, pinning her to the ground. She was trying to get the girl to stay by any means, so when Touka still didn't stop trying to leave, she bit down again... on her sister's neck. She bit through and destroyed Touka's windpipe, killing her near-instantly.

When Touka started screaming, her ten-year-old sister Ayano and their parents started running to the room. As it happens, they entered the room right as Kuruta killed her sister. As expected, they freaked out. Kuruta had an insane look in her eyes and started running at them, looking for comfort. Her family members were now deadly afraid of her, so they ran away... which only made Kuruta angrier. She quickly ran towards their massive kitchen and got out a knife and started finding the others. She quickly ran through the house, just looking for anyone that could comfort her, anyone that she could be with. Eventually she found them in the master bedroom. As soon as she saw them, she jumped up to her mother and embraced her in a tight hug. The woman was, of course, terrified, and tried to pull free. As soon as she realized this, Kuruta tried to injure her mother so she couldn't leave, stabbing her in the back with the knife. She was a little too accurate, however, and destroyed the woman's nerves, killing her too.

Once more, her father ran. This time, Kuruta acted quickly, pulling the knife out of the dead woman and throwing it after him. She hit the back of his head, destroying his skull as he dropped dead. The ten-year-old Ayano was now the only one that could offer Kuruta what she wanted: human contact. But the terrified girl could only stare at the dead bodies of her parents. She was completely frozen over from fear, so when Kuruta pulled her into a hug, she screamed loudly. Trying to shut her up, just wanting to enjoy the comfort, Kuruta held a hand over the girl's mouth. However, she kept this up a little too long and a little too tight, causing the girl to choke and die.

Her family now dead and no one around to comfort the girl, Kuruta just didn't know anymore. After a couple minutes, she realized what she had done and started hysterically crying. Her episode of craziness being over, the girl was now dealing with 4 dead bodies in her house and the knowledge that she was the one that killed them. She retreated into the corner, crying and bobbing back and fourth. Eventually, the police arrived, presumably because her neighbors had heard the screaming. They found the 4 bodies and crying girl... and had no idea what happened. Kuruta had hidden the knife under the floor, mainly because she never wanted to see it again, so her finger prints were never found. Completely confused at the family murder that had taken place, the police focused on finding a new home for Kuruta, quickly finding her a loving foster family that wanted to help her get over what happened.

However, a couple years later, Kuruta snapped again. The now ten-year-old girl had been trying really, really hard to stay together with someone at all times, since she remembered what would happen if she didn't. But at some point, while she was cooking with her foster mother and looking at the fridge, the woman went to the bathroom. Kuruta hadn't realized this and upon realizing she was alone, she went crazy. Now that she was slightly faster and stronger, the deaths were terrible, Kuruta clearly trying to keep them close. When the police arrived, they found a mother with both her legs cut open down, causing her to bleed out, a father with a throat that was cut open, most likely by someone hanging on his back, and a boy with a stab wound in his chest and cuts all over his arms. Once more, the knife was nowhere to be found and Kuruta was crying in the corner.

This time, the search for a foster family was a lot harder, with Kuruta eventually ending up with a single woman. The girl followed her new mother everywhere, not once letting her out of sight. This went well for years. The woman's colleagues found it weird that the girl followed her everywhere, but she didn't mind. Kuruta, meanwhile, did everything she could from going completely crazy. It's quite possible that she would never have gone insane again... until a certain letter found its way to her. When she was 16 years old, Hope's Peak Academy had somehow found out about what she had done and invited her to the school as the Ultimate Yandere. Kuruta's foster mother didn't understand any of this and tried to get Kuruta to explain what was going on. She, of course, couldn't tell her, saying that she was afraid the woman would leave her if she knew. The mother threatened to leave anyway if she didn't learn about it, and this made the girl snap. As soon as she was finished, she ran off towards Hope's Peak Academy, crying hysterically yet again. When the police arrived, they found the woman's head and torso hanging from the ceiling by her neck with a rope, while her cutoff limbs were scattered around her.

At Hope's Peak, Kuruta tried to isolate herself at first, making sure she doesn't grow too attached to anyone. Eventually, however, a boy named Kaguya tried to get close to her, not knowing about her secret. She had been calling herself the Ultimate Best Friend in order to try to conceal her past, but that only made the boy want to find out more. Thus, he tried and tried, and eventually, Kuruta couldn't let go anymore, wanting to be close to him at all times. Of course, it was inevitable that he couldn't fulfill that promise and was eventually found in his bedroom with a crying Kuruta next to him. His torso was on the ground with all limbs and the head torn off, seemingly by rope attached to each of them.

From that point, Kuruta tried to get along with as many people again, hoping that that would prevent her from being alone and insane... but the Ultimate Yandere is not stopped that easily.