Megumi

Megumi studied at Jujutsu Academy, she was a near-special class sorcerer, she was Gojo's favorite, and her best friend was Yuji, and she and Yuji were always together.
However, Yuji knew absolutely nothing about Megumi's past because Megumi was always reserved, closed off, almost never talked to anyone, and when Yuji asked if he had ever liked or dated anyone, Megumi never answered.
One day, Megumi asked Yuji to fetch an assignment from her dorm. Yuji went, but when he arrived, the dorm was all decorated in black and gray. When he opened a drawer, he saw a picture of a girl—a beautiful girl, but as a child—next to Megumi, also a child. It was a childhood photo of Megumi. Yuji found more photos of the same girl as a child and teenager, and he was shocked because if Megumi had photos of that girl, either he was dating her or she was someone from his past that he loved.
And then Yuji found a letter that said
"Hi (your name), it's been a long time since we last saw each other, hasn't it? I remember our childhood, when you used to annoy me every day. You'd come to my door just to drag me away to play. You know, one day I want to talk to you so that our fight ends because I love you and I didn't want to fight with you that day, and I know I won't deliver this letter."
Yuji was shocked because he knew that girl, since it was you, another girl who also studied at the Jujutsu school, but you had never spoken to Yuji. However, Yuji knew you because you got the highest grades and were also one of Gojo's favorites. Yuji read that letter and wondered if Megumi loved you. Yuji was going to play his role as best friend and try to understand what the fight was that made you and Megumi stop talking to each other and try to get you to reconcile. Yuji read that last part, "I still love you," and that motivated him. If Megumi still loved you, Yuji was going to help him, and the first step would be to befriend you because you had been his love.