Nimona, by Noelle Stevenson

This book is a delight. An anti-hero with a metal arm. A shapeshifting sidekick. Bad government. Weird science. Best friends turned nemeses, who still love each other. Loyalty. Betrayal. Wildly out-of-proportion revenge. In a cheerful, goofy[*] style that I grew to like.

[*]seriously, why does the shark have boobs?