For these purposes, “asexual” or “ace” refers to any character who is or could be (with reasonable evidence) anywhere on the asexual spectrum. In other words, graysexuals and demisexuals count too! Also welcome are asexual characters who simultaneously identify as gay, bi, trans, or anything else.
Bonus points for anyone who finds an asexual character who is of color, disabled, or from an under-represented culture/religion/nationality. Sadly, those are some of the hardest to find.
Luna Lovegood from the later Harry Potter books has been thought to be asexual, so I thought I’d mention that. The later books could conceivably be considered YA since the themes are darker. They include:
Harry Potter and the Order of the Phoenix (Book #5) by J.K. Rowling
Harry Potter and the Half-Blood Prince (Book #6) by J.K. Rowling
Harry Potter and the Deathly Hallows (Book #7) by J.K. Rowling
The YA book When the Black Girl Sings by Bil Wright has a protagonist, Lahni, that I think may be ace. At fourteen, she explicitly states that she is not interested in relationships and shows no interest in romance. The topic of asexuality was never mentioned, but anyone looking for an African American female character in a non-romantic book will certainly enjoy this.