When it comes to diverse books, who makes them diverse? And by "who", I mean which characters?

When I was recording the diverse books I read last year, I was only taking note of those where the/a protagonist/narrator was from a marginalised group. This was down to a conversation I had on Twitter with an author who said (and I'm paraphrasing) that she personally didn't consider a book to be diverse if the book wasn't told from the point of view of the marginalised character, as it doesn't get into what it means to be marginalised, and I could see her point.

But this year, I'm starting to think that's not necessarily the case. Take A Quiet Kind of Thunder by Sara Barnard for example. It's a diverse book anyway because the protagonist, Steffi, has anxiety and selective mutism, but even if she didn't, because the love interest, Rhys, is a biracial deaf person, I would still say it was a diverse book. There's no escaping that he's deaf, Steffi has to speak to him in British Sign Language, and there are several discussions about how he struggles and how difficult it can be to navigate the hearing world. He doesn't narrate the story, but tha fact that he is deaf is pivotal to the story as it affects all communication with him. As I said in my review, I would have preferred A Quiet Kind of Thunder to be dual narrated as it would have been great to see these things from his perspective, but I don't think the fact that it's not would mean the story wasn't diverse had Steffi not had a mental illness.

But A Quiet Kind of Thunder is a romance, and that makes Rhys a main character even if he's not narrating, right? So perhaps it's down to whether or not the marginalised person who isn't the narrator is a main character instead of a secondary character? Steffi's best friend, Tem, is Somalian, but she's not a main character, as it's a romance, so she wouldn't make it a diverse book right? But if a book was about friendship, and one of the friends of the narrator was from a marginalised group, that would be a diverse book, right? Because they are a main character, if not the narrator.

Of course, I'm aiming to read books where the narrator/s are marginalised characters, but when it comes to recording the diverse books I read, should I or should I not be recording books where the marginalised character isn't the person from whose perspective the story is being written from? Basically I'm asking whether you would count them? It's because of this that in my Diverse YA & MG 2017 UK Releases list, I also note books featuring prominent marginalised secondary characters, in case other people consider those books to be diverse.

I myself have decided to count every single book that features a marginalised character this year, even if they're only a secondary character, so that at the end of the year, I can look back at the number of books I've read that featured marginalised protagonists, other main characters from marginalised groups, and secondary marginalised characters. But it's interesting to think about what books count, and what books don't. I'd be love to hear what you think!