Adeline Radloff's Sidekick (2010) is about, well, a sidekick. Katie Holmes (no, not that one - a joke that's just barely on acceptable side of annoying) is a teen adoptee, living with her foster mom and and Finn. Finn is Bruce Wayne. Mom is Alfred. Katie is Robin.
There's a little more complexity to it (there's a more Alfred-y Alfred, but he died somewhere in the past, for example), but that's the book in a nutshell. Finn, unlike Batman, has actual superpowers as well - he can fuss around with time, although there are various limitations and side effects that are introduced as the book goes on. Katie is remarkable because, besides Finn, she's the only person that isn't frozen solid when Finn does his time-travel mojo. Which means she can help Finn move stuff around, save lives, fight evil, remember to eat a sandwich, and, you know, be a sidekick.