You know, it's interesting...
When I start a new series of novels I often find some characters seem lacking in any real character and it takes time before I can hear their distinctive "voices" in my head while I read the novel.
But I must say, right out from the gate - I could hear distinct voices for each character. I thought the characters developed very well through the novel.
The whole Grimm/Ransom thing though, I've seen that exact interaction type done many times before in TV and books, but I still find it fun.