Theodore Nott did not plan for his best friend to find out he was a Death Eater during the attack on Hogwarts in their sixth year. In fact, his plan was to keep it a secret from her for as long as he can. Given their situation at that moment, it was justifiable that he'd panicked. But what wasn't justifiable was his mistake.
With shaky hands and tears streaming down his face, the boy had uttered the incantation. He had intended for the spell to erase only the part of him that she despised so greatly that she shook in anger when she saw the mark on his forearm, but in the current state of the castle filled with destruction around them and the regret that clawed at Theodore's mind in that moment, he had instead erased all of the many memories they had shared with each other.
But he had only realized that when he came across her again and saved her life.
【 THEODORE NOTT X OC - DH 】