The first four volumes is a great warm series. The story is new, fun and mysterious. Noelle Stevenson definitely puts her great story-telling abilities to good use in this series. All characters are cute and fun in a natural way. This reminds me of Nimona also by Noelle. I don't know how to describe it, but the story always move in some peculiar pace that's unpredictable yet fun to read.
There is no single one most prominent character in this series. Yet all characters shine beautifully. The last series that accomplish this I have read is Runaways by Brian Vaughan. Unfortunately, like Runaways, this series made a a bad turn when the original storyteller left the team, as the vol.5 and vol.6 are not good at all without Noelle Stevenson.