
I thought it was either? Like I've read manga before that distinguished between a character using a gender neutral pronoun and a gendered one. Plus in some shows too like hunterxhunter where Killua refers to his trans sister with her preferred pronouns while his family refers to her with male pronouns.

Pretty sure it can be either. If you refer to a third person, for example, it can be ano hito=that person, kare=him, kanojo=her. Same with 1st person, boku and ore is more masculine and often used by male, but watashi is more gender neutral. Japanese also tend to drop the subject when it can be inferred from context, so sometimes you may not be able to tell if a person's talking about a guy, a girl, or a turtle ┑( ̄Д  ̄)┍
Can't help ya on your question tho

Yeah, and on a thought bubble in another panel, Riku said "this guy might be a weirdo who likes wearing girl's uniform, but he's a good friend."
I think it was translated using they because the subject was omitted. The actual sentence in Japanese might have been, "Mizutani isn't my girlfriend. Just a friend." which might sound a bit awkward w/o a subject.
Another example where the translator used they is when Ichigo's friend was talking about their first encounter. He said, "they're cute, with the little body of theirs" and Ichigo thought he was referring to the girl. But you can't be sure with the way he said it. He might actually be talking about Riku.
So I hate to break it to those who think otherwise, but Riku most probably didn't use the right pronouns ╮( ̄▽ ̄)╭
Edit: why the downvotes? guys I'm literally just asking if he's being respectful or if the translators are the ones being respectful. Not saying he shouldn't use his friend's preferred pronouns...
Is he using the correct pronouns for his friend or is the translator team the one doing it? I really hope it's him but given his comment in the first chapter "even though you don't look like anything but a gay dude"..idk.
I mean if they have the kinda relationship where his friend's fine with him poking fun at that, that's cool of course. I just see it as a common "joke" in yaoi for the transwomen to not be taken seriously and is treated as a cross dresser instead :(