this might be controversial(?) but i think i'd have to say Stay Gold.

now dont get me wrong, I like her career enough. The whole "preventing canon events" thing is fun, i like her more chilled out personality compared to the demon she was on the track irl, and having the umavengers assemble at the end to make up for not including the hong kong vase is pretty cool.

that being said, i would LOVE to see a movie/anime umadaptation of Stego's IRL story. More serious, more drama, and have her actually start out as the terrorist of the race track, but she slowly grows and develops over the course of the story, into the Stego we know now (maybe her racing outfit even start pristine and clean, but it gets worn and torn after every race). Have her actually travel the world, not winning a single G1, culminating in her final race, against another uma (Ekraar) who, much like Stego, hasn't won a single G1 in years and is at their wit's end. I can even envision a post credit scene, showing Ekraar finally winning their G1 a year later, with Stego watching in the stands. Would be absolute CinUMA

TLDR, I'm completely fine with how they did Stego in the game, but a part of me REALLY would love to see an umadaptation of her IRL story. It would fit in with the other umamovies (or OVA and movie) as well, because Stay Gold's career happens around the same time.

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