How do they work, do you add a retainer cards strength and icons to the attached character?

The retainer trait is more about flavour than functionalty, it´s just a trait like any other trait. Most (probably all) released retainer cards these days are attachments, so they work just like any normal non-retainer attachemnt would do.

Thus if you add e.g. Court counsel ( www.tzumainn.com/agot/cards/card.php ) to a charachter, it adds an intrigue and a power icon to this charachter.

It might help answer your question if you could give an example of a specific card you're looking at. "Retainer" could refer to a couple of different things in the game (even more if there's a translation going on) so a specific card would help make sure everyone is thinking of the same thing.