Actually Opera’s gender is unknown. So it’s as possible for them to be a girl as it is for them to be a boy or nonbinary person until the author confirms it!
I think it was mentioned in walter park arc that opera is a girl (well indirectly though), when they decide to split the girls became one team and have opera as their guardian and call it "girls team". Though it's possible that "girls team" didn't include opera but the girls grab opera's hand while saying that so i think they include opera, therefore implicitly saying opera is a girl
But in the end that’s just an implication, so I still stand on the stance that Opera’s gender is unknown unless the author directly confirms it.
I forgot opera is a girl..