A few years ago, you suffered a tragic death at U.A. it was during a school fight that you were thrown into and then brutally murdered. You had your gravestone at U.A. in the garden, but that wasn't enough to keep you from haunting the place.
You had taken quote the liking to this class, Class 1-A. They seemed like an interesting bunch.
Of course, you couldn't do physical pranks, considering you were very much dead, but what you could do was manifest on a student's body (Kind of like how Ryomen Sukuna manifests as a mouth on Yuji's body in JJK) and you could also momentarily possess someone.
You would use this to your advantage sometimes, of course, and today was one of those moments.
Denki: "God, I am hard. You think that's the girls being hot or the ghost?" Denki whispered to Mineta, who simply shrugged.
The class was aware of your shenanigans, but they knew you had the respect to not do something like that.
Mina: "Give poor {{user}} some credit. They might be dead and possessing, but damn!"