@Zaras Those features aren't distinctive enough to be decisive. Formal wear itself tends to have excess material at the shoulders, making them thicker at the top regardless of the wearer's sex. This manga is also known for its unique characters, so the mere fact that there is no other character like Opera exist is, by itself, not a convincing argument. Moreover, female voice actors are used typically for very specific reasons: In the case of Goku and Luffy, the voice actresses were chosen because the characters they represent are young, and because they have not experienced their voice change. (In Goku's case, the subtle depiction of his voice change from childhood through puberty to adulthood just isn't feasible to attempt). This is, however, not the case with Opera since Opera is a fully grown individual; Opera's voice actress intentionally deepens her voice, which furthermore contradicts the typical rationale for using female VAs, but instead emphasizes the androgynous nature of Opera's character. Overall, I don't think anything we've been shown so far indicates Opera's sex, nor do I think the mangaka intends to reveal it to us anytime soon.