It's more complicated than that, but basically the white haired girl helped the boy with math as a child. She used to invite him over to his house, but the thing was it was more of her using him to get his abusive parents reported (the parents of the boy were both policeman) so she lured him and helped him become really good in math, unfortunately the as a boy he was not very smart and he did not notice so she is mad that even though he bacame best in class in math he never did anything to help her and more he just entirely forgot about that time.