For some reason having a non-white protagonist means this seems to be the first thing everyone asks about, so I will tackle it up front.
Aside from Karo's own mixed-race background, I've left the racial background of characters deliberately open.
This has had one very interesting result: the number of reviewers who have brought their own background to it. The American reviewers tend to insist the gang members must be black, while the protagonist is white. British reviewers, particularly those that know the area it is set in, reverse that.
I was actually asked why Karo 'had to be' mixed race. The answer was simply because I wanted her to be the average for her area, and the area of London where the story is set is majority non-white. Blending in is her first and only defence, and it would be difficult to be overlooked by good or bad guys if she wasn't average.
When it comes down to it the focus of Going Under is less race than class and poverty, and the unfortunate fact that in today's society, once you fall off the ladder, getting back on can be nearly impossible.
And it has a cat in it.