What Alexstarfire said. If you have any sort of RWBY merch yourself you might consider using that to try and establish common ground. If nothing else you can test the waters with something like “cool shirt, is that a show you like?” – that gives you an out in case they’ve never heard of it and just got it secondhand or because it looked cool.

Walk up to them and say hi like you would anyone else? You could ask who their favorite character in the show is.