From Fantasy Flight Games:

we have developed an expansion to the AGoT LCG Core Set, and we’re calling it the: Kings of the Sea. Kings of the Sea was developed with three major goals in mind:

The first of these goals was to bring House Greyjoy back into the LCG environment. From this perspective, the expansion consists of a Greyjoy deck that is similar to the out-of-the-box decks that you’ll find in the AGoT Core set. With a mixture of both new cards and old school Greyjoy reprints (some modified and updated for the LCG environment), House Greyjoy can once again stand proudly alongside (or, if you prefer, against) Houses Stark, Lannister, Baratheon, and Targaryen in the LCG environment.

Secondly, I insisted on using the design of the Kings of the Sea expansion as an opportunity to address some of the context and synergy concerns noted earlier. In this product, you’ll find numerous King and Queen characters, some new Kingdom locations, and a spattering of Military Battle, Intrigue Gambit, and Power Struggle plot cards. Additional use for crested characters is also prevalent (particularly Holy and War), and a major black hole in the AGoT LCG metagame, a realistic check on powerful locations, has been confronted.

Third (and not unrelated to the context and synergy discussion), we wanted this expansion to have something for everyone, even if they’re not at all interested in House Greyjoy. Thus, each House gets at least 1 King or Queen, and the plot deck (with the exception of an important House Greyjoy only plot reprint) along with many of the neutral cards were selected as cards that could potentially be useful building blocks in the widest possible array of decks. So even a die-hard Lannister player (there aren’t really any of those out there, are there?) should be able to find something of use in this set.