esper and seraph are both spirits, which are sexless/genderless in nature (their true form is a blob of magic), but theyre both pretty different from each other. spirits express/think of gender differently than we do. theyre still genderless, but they can align themselves with femininity or masculinity (on rare occasions) or w/e.

first of all, esper is mostly neutral. he uses male terminology as a convenience, since he was created in a male form. he doesnt really see himself as a man or a woman. just as a spirit. he feels comfortable in loose, flowy clothing, so hes okay with dresses and skirts. he doesnt like being called a girl especially, because he was a counterpart to gaea and he doesnt want to be reminded of her.

seraph, however, is different. he is masculine and wants to see himself as a guy. its pretty rare for a spirit to really ID like that, but ser does. he doesnt like girly clothing and wants to be seen as a man. hes a sexless blob of magic, but he sees himself as a dude.