@Missi
So please, tell me again why I shouldn't say that it's their gender ?
Well first of all choosing to not identify as a gender is not a gender itself, much like identifying as a goth isn't a gender.
They uses "They/Them" pronouns in the translation
Now I can't speak with any authority on this, but Japanese pronouns tend to be either masculine or feminine somewhere along a grey-scale where some are more extreme than others but with no truly neutral options.
Maybe the raw is simply using Opera's name instead, and constantly repeating it would be awkward to read so the translators opted for a neutral pronoun.
Lastly, maybe the characters don't know Opera's gender either and are just politely avoiding direct references to it. That doesn't mean it's a reflection of Opera's own self.
So until we get an official word on it, I will think of them as non binary like so many peoples do, because it fits
That's like, your opinion, dude.
No harm in deciding your own head-cannon if things are left purposefully vague.