Yeah. So, my assistant is my height, so meaning that in heels (which I prefer her to wear for the costume) is taller than me by a by about 3 inches or so.

How would that look as a performer? Odd?

For a few years I had 2 Girls in my show that were taller than me. I'm 5'11", and they both were several inches taller than me - they both were playing basketball. I never heard any comments from anyone about them being taller than me. The only draw back was that they were both to tall to fit in my Zig Zag !
My bet is that everything will be fine.
Bob.

I had an assistant that was taller than me, but the only illusion she had to get into was the Victory Cubes. The tights I own for the costume were a little short, but she wore socks to cover up. This was a kids show. Other than that, the show was fine.

If you still want her to wear heels, make sure they are black character shoes and not nude. The nude ones will make her legs look even longer. Try to get the shortest heel possible. I believe character shoes can have as small as a 1/2 inch heel. Also tights that have a seam running down the back of the leg create the illusion that she isn't as tall. Hope this helps!