Ah, yes. My 12 year old DD is exactly the same. Although she's like on the AS and that can be a classic trait.
Drives me insane actually. I asked her outright once, how she'd feel if someone thought she smelt bad or had bad breath and she shrugged "it wouldn't bother me"
I'm SURE it would in reality, but in theory, she doesn't get what the fuss is about. She has two showers a week, which I would rather was every other day, but I guess I pick my battles. Same with changing underwear everyday and washing her hair. She doesn't think there's anything wrong with having greasy hair.
So I don't have much advice really
I think at this age though, a bit of a tough love/cruel to be kind approach, might be the only way slightly forward. For example, if my DD does smell bad, I will tell her. Not in public or in front of anyone and I don't say it in a nasty way, but I will say "xxxx, when did you last shower?" and then just give her that look. I do feel bad, as I don't want to hurt her feelings, of course. It's 50/50 whether that gets her in the shower, so sometimes I just run it and tell her she needs to go in, like I'd tell her to do her homework, tidy her room etc.