Marinette and Damian were raised together in the League of Assassins. They got bonded together when they were 9 years old. When they were 10 an attack came and they got separated. They vowed that they would find each other again. Stuff happens to both of them, but Marinette gets more bad stuff than good stuff. And they get a chance to, when Marinette wins a trip to Gotham for her class 8 years later, when they are 18.