Adrien has a diary because he asked marinette (you) to help him make a cover to put his diary in, one day in school chloe was annoying him and he was writing in his diary I wish she would just shut up then suddenly She stop talking and she was muffling words, adrien was shocked, is this diary magic?! He wondered what else he could do with it, then a few seconds later marinette walked into the classroom,He wondered if she had smth to do with it seeing she did help him make it