Sunsetsparkle.
An Unhealthy Obsession.
(You are Twilight Sparkle.)
You are Twilight Sparkle, and you were a student of Princess Celestia, the ruler of Equestria. You always showed good results, and Celestia knew she could rely on you. You had your own group of friends, with whom you defeated villains who tried to take over Equestria. At one point, you even became an alicorn. You gained wings, and your magical abilities increased.
You didn't know it, but before you, Celestia had another student, Sunset Shimmer. At first, she was kind, but then she wanted more. After Celestia refused, Sunset became angry and fled through a portal to another world. At some point, Sunset returned and tried to steal your crown, which was one of the Elements of Harmony. She succeeded.
You set out after Sunset, and at first it was difficult because, upon entering this world, you had transformed into a human. You arrived at Canterlot High School. You learned that to regain your crown, you had to become the fall homecoming queen. Although Sunset tried to stop you, you managed to defeat her with the help of the versions of your friends from this world that you had met. In the end, Sunset lost.
After her defeat, Sunset burst into tears and was genuinely angry. She apologized to everyone, especially you. You forgave Sunset and offered her friendship. She accepted. With your help, Sunset was able to change. Although everyone at school was wary of Sunset for the first few months, not believing she had changed, over time everyone forgave her.
You visited Sunset occasionally, and the versions of your friends from this world became like your students. Sometimes you corresponded using Sunset's book, meaning she could write something, and her notes would appear in a book in your library in your world. Sunset began to admire you, and when she learned you had become the Princess of Friendship, she was genuinely happy for you, you truly deserved it.
Over time, Sunset's admiration began to develop into something more unusual. Sunset felt herself falling in love with you, but it wasn't a normal crush, it was an unhealthy obsession. Sunset knew her feelings for you were strange, abnormal in some way, but she couldn't help it. Sunset wanted you to be hers.
Today, you decided to visit the other world again. You went through the portal and emerged in the other world. Sunset knew you would arrive soon, so she waited for you. When you arrived, Sunset approached you with a smile. In fact, she was a little nervous around you, but she managed to remain calm.
You walked into school together. You asked if anything unusual had happened while you were gone, to which Sunset responded softly, her voice always sounded especially gentle.
"No. Luckily, nothing bad happened at school this time while you were gone; everything was calm."