Marinette is Bakugo's sister. There are quirks in this world but they are extremely rare outside of Japan. Marinette has a weaker version of bakugo's quirk being able to make explosions from her sweat that burn a pink color and are more force than heat and produce less smoke, but she doesn't use it very often to not draw attention to herself for having a quirk. She lives in Paris with her aunt and uncle because just before she started middle school she got the opportunity to go to a special fashion school in France so she moved in with her aunt and uncle and dyed her hair from its natural dark blonde to a deep blue to match her aunt and was passed off as their daughter. Everything goes as in the shows until about just after Bakugo was rescued (just pretend like the fight between all might and afo hasn't happened yet) and about some time in between episodes 13 and 14 of season 3 in the mlb timeline when marinette gets a call from her mom telling her about what had happened with the league of villains and bakugo. Hearing what happened to her twin brother marinette was about ready to transform and head to Japan herself but she stopped herself and kinda got some strings pulled by her friend from back in Japan and her online chess partner to have her class go on a field trip to Japan to see what life is like for the students of UA without the school having to pay anything as all expenses were paid for by UA themselves. (tho if endeavor randomly lost about 100k from his bank account, Shoto ain't gonna snitch.) With all of them having seen the sports festival, her class was really excited and lie-la started lying about knowing the one who won the sports festival (bakugo) so marinette decided to just keep her brother a secret for now, just saying that she knew someone in japan that was willing to let her and some of her friends stay there.