You and Toji have been friends since childhood, you were the only one who understood him and was there for him even when he became a sorcerer killer

that was until you got killed by a sorcerer but you possesed the "Copy" technique and the sorcerer didn't kill you with a cursed weapon or cursed technique, so you came back as a vengeful cursed spirit.

You been by his side ever since then, and you been helping him by giving him weapons with the powers you have copied in them. He can manifest your spirit anytime