A starter deck for every faction is included in the Core Set. You can make your deck with cards from a single faction, but like A Song of Ice and Fire , your games of A Game of Thrones: The Card Game Second Edition are filled with alliances. Every deck has the option to use an agenda to call upon the support of another faction. If you call upon the support of a faction by using its agenda, you can include cards from that faction in your deck free of penalty. The exception to this rule is loyal cards. Loyal cards include characters unshakable in their devotion to their House, locations intrinsically tied to a faction, or events that represent tactics used only by a single faction. Loyal cards can never be included in another faction’s deck.
I think we can all agree that Loyal is a mechanic vital to Second Edition, for both mechanical and thematic reasons. You don't want to see one faction constantly showing up in other factions' decks because of its dominance in some critical aspect of the game; nor do you want people like Tywin Lannister or Doran Martell, figures wholly committed to the prosperity of their houses, willingly pledging their support to other factions.
At the same time, though, I find myself quite apprehensive toward this element of the game. I believe it can be used correctly, and hope that it works as well as it is intended; however, as much as I love expanding card games, I consistently come at them from the vantage point of a story-lover. I just can't help it. Whenever I play a game that has a sufficient dedication to theme woven into its mechanics, and A Game of Thrones is about as thematic as they come, the actions taken by each player, from plots to challenges to events, automatically take shape in my head as a flavorful narrative unique to the match at hand. That being said, there are situations that can break me out of this spell, and loyal has the potential for this, depending upon how it is used, and on whom. I am referring specifically to character cards here: though I imagine there are other card types for which this could apply, I find I am not as concerned by them. I do think that each house faction ought to have its motto appear on either an event or plot card (preferably with consistency either way), and that these cards should be loyal.
More than anything, I am thinking about the role of Eddard Stark at the outset of the books. In one of the earliest chapters he is recruited by his friend, King Robert, to become the new Hand of the King. From a flavor standpoint, I believe this makes it clear that it should be possible for a Baratheon player who has the support of House Stark to marshal Eddard. But as the Lord of Winterfell, if he is not loyal it is difficult to imagine who is. Thus I believe, putting aside the mechanical restrictions intended by the loyal keyword since we know very little about this beyond what has been stated, the flavor concept of loyal should really come about for characters who are devoted to their house above all else. Tywin and Doran especially come to mind, as well as all King characters. Ned, on the other hand, while having responsibility for House Stark, honors the words of his house: "Winter is coming," and as such, he values the realm and its safety above all things, and should be able to support another house.
There is of course the strange imagery of Eddard Stark appearing alongside Joffrey Baratheon or Jorah Mormont. But such situations can be justified, I think, with staying true to the canonical sequence of events being of primary importance. Thus every character who has aligned him or herself with another faction at least once in the storyline should have at least one character card that is not loyal. To use Tywin as an example, he assists Robert in the rebellion against the Mad King, and has indebted the realm to Casterly Rock beyond any possibility of repayment. But more often than not, we should see Tywin playing for House Lannister and no one else.