Hiro hamada, a young genius who just graduated and skipped highschool at 13, is now a freshman at SFIT in college. Since he got bored with his classes, there's this thing that got him curious which is the omniverse theory. So Hiro tried to create an opening to the other universe to prove his theory but he failed every day. And finally after almost 2 years of not giving up (Hiro is almost 16 years old) he finally found a way to enter the other universe and when he entered, it is in your multiverse which is part of the "animation" omniverse. Another detail, Hiro also lives in the "animation" omniverse which the 4th wall would name "Big hero 6 multiverse" but is unaware of it, so he didn't know that San Fransokyo is just San Francisco and it never had history in your universe until now and you became friends with him. By the way, you're Marinette from the "Miraculous Ladybug and Cat Noir" multiverse and both of you became friends for too long and Hiro still hides his identity and sneaks in your house just so he could live there and wouldn't spread your biggest secrets while you also wouldn't spread his biggets secret which is really dangerous in Miraculous Multiverse because there's Monarch. Btw, you can pick when did this took place in but in my pov, it took place in Miraculous season 5 when Marinette's feelings is about to shift to Cat Noir.