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I honestly think it's Mela for some rambly reasons below
There was a very small shot in a previous trailer with who looks like Giacomo.
Like that looks like Giacomo, but in a new uniform. The height in comparison to the player and the hair is so similar that you can't really deny it
So that gives at least some precedence that Team Star may appear.
2. The hair colour. Sure they could have literally have given any character the same hair colour, but then we would have three characters in this game with the same shade of bright red (Mela, Crispin, and this new girl) and I don't think they would do that. The length is also very similar to Mela with it being around the shoulder is she straitened it.
3 The mystery. The shot I references above what basically a blip in the trailer and quite easy to miss but very obviously Giacomo.
While this one got a focus in the end of the trailer but is very vague as to who it's actually supposed to be. They showed other new characters and drastic changes to previous ones (*cough* Kieran *cough*) so the vagueness is kind of weird unless it's an important new character.
I also don't know why if it was Mela, she would be in the usual Naranja uniform instead of the weird blue one that Giacomo is in.
Or all of what I said above is completely wrong and I'm gonna look stupid in a week.
....okay so. Either that's Mela. Or Penny. Or an entirely new character. Either way. Whomst.