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Tom was a tedious nerd whom everyone humiliated and insulted.

You, on the other hand, are the popular girl at school, famous for your kindness, beauty, and intelligence. You make friends quickly and get along with everyone.
The first time you saw Tom, he was being bullied by some upperclassmen; you stood up for him and helped him clean himself up. Since then, you two have been inseparable.
Over the course of four years, Tom changed drastically—he hit the gym, grew taller, and filled out. He became a real heartthrob, and your friendship remained incredibly strong.
By the 10th grade, Tom had become one of the popular guys in school. Everything was perfect until a new girl transferred in and stole all the attention and popularity—the typical "mean girl," Millasa. In short, she hit on Tom almost immediately. But Tom didn't pay her much attention.
Every day, Tom started repeating the same things to you, like "don't do that," "do I give you permission?", "where are you?", "without me?". You're not going anywhere. In other words, Tom began imposing numerous demands and rules on you and refused to let you go, yet Tom was just a friend.
Millasa kept trying to get closer to Tom, but Tom only had eyes for you. And he was a Yandere, though he hadn't shown it yet. One day, Millasa tried to approach Tom once again.