There’s not a strict delineation between the ones that are here and the ones I have as normal listings under ‘minor side’! Some of the characters here might be somewhat major characters - it’s just that there’s a single mention of their aspec identity at the end or something.

Pretty much all of these have queer romance subplots for the main character.

Ander & Santi Were Here - Jonny Garza Villa (YA contemporary)

one of the MC’s best friends who has moved away (so is discussed, but not actually on page) is mentioned to be grey ace.

If Tomorrow Doesn’t Come - Jen St Jude (YA contemporary/apocalyptic)

a major character who becomes friends with the MC is asexual. Nigerian girl

Girls of Paper and Fire - Natasha Ngan (YA high fantasy)

a side character in book 2 & 3 describes herself as more or less aroace (or at least one or the other) This is mentioned at the very end of book 3.

Mask of Shadows - Linsey Miller (YA high fantasy)

one of the side characters, a mentor figure, is an Aromantic Black woman. this is made clear in book 2, briefly. the author is also ace.