Flipped

Endeavor was furious that all his times he tried to make his and Rei's quirks fuse in a kid don't work. One for all heard and he gave Endeavor a quirk mixing serum. He knew Endeavour was having twins and the price of the serum was one of them. Rin Todoroki is Shoto's twin sister who feels all of his pain. However they are both flipped. Shoto has red and fire on his left however Rin has them on her right, the pain also flips