As you can see from this tweet and confirmed in a later tweet, Voice Actress, April Stewart is going to voice a character in Book 2 of Korra. (https://twitter.com/Aprilstewart)

The fact that she retweeted this confirms it I think.

In case you do not know her, she is best known for playing multiple character in South Park including Cartman's mom.

In terms of characters she could play, most likely it could be Kya, Tenzin and Bumi's sister, though she could play Unalaq's daughter or a new character all together.

Her first tweet may or may not also give us an idea of when book 2 is coming or else when we will be getting some more info. Though from what we experienced with Book 1 we know the voice Actors are pretty much only allowed to speak about aired episodes.

If it does indeed air in April that is more or less exactly a year since Book 1 Aired and would make sense given the wait between books of ATLA.

Not confirmed yet, but a good sign.

What are your thoughts ?

Amon The Checkerboard Champion