Yaomomo Becomes Santa Claus (Through Accidental Manslaughter)
It was the night of Christmas Eve in Japan, and Class 1-A was celebrating with a full-on party, down to even wearing Santa outfits. However, they hear ruffling outside of their dormitory, and decide to go take a look and see what it was. Upon noticing an individual on their roof, one Katsuki Bakugo, in his usual brash way of course, tells them to get off of the roof and causing the mysterious person in question to slip and fall off from the shock. Upon taking a closer look at who they were, everyone's faces are filled with complete dread as they realize; they killed Santa Claus.
This was bad. This was really bad. They've not only taken an innocent life, they've taken the innocent life, and have irreversibly ruined Christmas for all the children around the world, along with cementing themselves on the Naughty List for years to come. Many were already questioning dropping out of U.A. High and putting an end to ever becoming heroes for committing such an unspeakable crime, but Momo Yaoyorozu, with all the confidence she could muster up at the moment, silences everyone by saying that she will take the mantle of Santa for this year at least. Since she can't do it alone, everyone agrees to help and in no-time they're up and away on Santa's sleigh to deliver all those presents while trying to keep Aizawa busy from questioning why they weren't at the Heights Alliance. They're split into two groups, one of which accidentally takes the magic bag with all the gifts with them and leaving the group led by Momo bare-handed, forcing Momo to use her quirk in order to create gifts based on what the list she has says. Thank Kami for the cookies and milk. After 24 hours of stress and hard work, the gang returns back to Japan satisfied that at the very least they saved Christmas for now and have re-proven themselves worthy of being heroes.
New Year rolls in, and Momo begins noticing changes in her appearance. Her hair has become white, she has become chubbier, and she can't go a single sentence without hollering "Ho Ho Ho!". As she and everyone else begins questioning what's going on, an elf from the North Pole comes to visit and reveals the shocking truth; Momo is the great-great-great-great-granddaughter of Santa Claus from her paternal side. This is shocking to Momo because not only was she complicit in killing her ancestor, but now with him gone she has to bear the torch of responsibility that comes with spreading cheer and joy at a specific time of year. However with support from her friends, particularly Kyoka Jiro and Izuku Midoriya who himself is going through something similar (and who's also trying to grapple with the fact that he's part-elf since his father apparently works at the North Pole as an accountant of sorts), she musters up confidence and picks up the role, it even becoming her identity in heroics. Her hero outfit changes to that of the Santa outfit she wore at the party, and she changes her name from "Creation Hero: Creati" to "Christmas Hero: Santa Yaomomo". While a more seasonal hero, the elves have no qualms with her working any other month of the year, which means she can still become a hero in the end, so win-win for everyone.