@miyako19
While that's true, it may not be a hard and fast rule, especially if the author's intent is to make the audience question. Opera being a girl would be way too obvious if she had the typical eyes.
Also note that several of the boys have those features (Agares, Chadruma). Opera is generally extremely androgynous, but I lean towards her being a she for these reasons.
1.) Iruma needs a mother figure equivalent. Sullivan is there as his doting (grand)father, leaving the slot for mother open.
2.) During this arc, Everyone split up into groups of three boys with a male guardian, three boys with a male guardian, and three girls... With a male guardian? No, I think their guardian would likely be a female. In this case, Opera. This manga does have a few instances of 2 guys 1 girl that I think is echoed here.
3.) Opera, Kallego, and Balam are somewhat reminiscent of Iruma, Asmodeus, and Clara. In line with that '2 guys 1 girl' thing I mentioned, I believe that Opera corresponds to either Amelie or Clara or a hybrid of the two (and their personalities are actually not as far apart as they appear on the surface. From Kallego's reactions, we know that Opera went at her own pace and probably just did whatever she felt like, similar to Clara.) That leaves Kallego and Alice, and Balam and Iruma (who have also had a few similarities pointed out.)
4.) Opera's mannerisms in some scenes are pretty girly, or at least make me think of a girl who's restraining herself to appear serious and composed. Like the thing with her ears wobbling and a slight blush when Iruma gave her the heartstealer smile.