So, the term "defending player" has recently been clarified to me that it is specifically the player is currently declared against. The player that would play claim, regardless of supporters. It's not a very common term, since its basically only used on CORE Stannis and any pseudo-claim cards...A significant difference from "The defender", while very similar this one basically seems to include supporters (as per the last line of the example of the supporting rules).
But i suppose its worth noting because its a term really not described anywhere that i can see. I kinda had to derive this from all the current cards with the term, and ffg's ruling on stannis and supporting players.
Are there other undescribed terms like this? probably too many to mention lol. What about "losing opponent" on the spoiled card "No Quarter", would a supporting players characters be legal targets?