[Corvina]

Himeka isn't an Ayu impersonator as far as I know. Well, she wasn't as I was following her 2006~2007.
She is a well known japanese model, former model of Ageha magazine.
She's a huge fan of Ayu, she already was at her time at Ageha.
I was so surprised when I saw the thread about her and her blog at AHS not long ago and somebody saying she impersonates Ayu. I laughed so hard! XD
It's possible that she's now an impersonator, but I highly doubt that's official.
I think she's just a big fan and gets her inspiration from Ayu.

You can only call somebody an impersonator when he or she dress and act like the idol to be recognized as this idol in public.
So Himeka really isn't one, she's still her own character.