I'm a new player. I have read various articles that recommend getting a 2nd and even a 3rd core set. Which cards are needed from these sets? Or, which cards are not needed from these sets?

Most cards in the Core Set come as singletons, so buying a second core set is an excellent decision. Not only does it give you some deck-building flexibility without adding new rulings or cards you wouldn't recognize, it gives you duplicates of very important cards regularly found in decks to this day; Flame Kissed, Power of Blood, Forever Burning, Golden Tooth Mines, You've Killed the Wrong Dwarf, Motley, Milk of the Poppy, Seductive Promise, Support of the Kingdom, Retreat, Distraction, Distinct Mastery, Westeros Bleeds, etc. as well as giving you the opportunity to keep 2-3 decks built at once, thanks to additional copies of commonly used plots like Valar Morghulis.

While there are a few cards you'd probably want 3 copies of (Golden Tooth Mines is an excellent example, and Westeros Bleeds is too if a heavy-reset deck is your flavour of choise), buying a 3rd core set tends to be reserved for completionists (like myself) or players who are either part of a small meta where they're unlikely to be able to borrow that 3rd copy, or supplying cards for a group of players.

I see. Are there any other must-have cards from the core set? I don't see any characters in your list.

Core eddard!

I see. Are there any other must-have cards from the core set? I don't see any characters in your list.

The CS characters that see the most play are probably Khal Drogo and Robert Baratheon. But the CE includes 2 Roberts, and most Targ decks that use CS Drogo play just 1 I think.

Some of the non-uniques are helpful, such as Knight of the Tumblestone if building Stark knights or war crests or Tully. There are probably some other non-uniques you may want 2 or 3 copies of.

I'd say that first starting out, you want x3 of all the basic resources for each house, and x3 all the non-unique characters that cost 2 or less. In general, the only unique characters I would want x3 of in any single deck are Robert and Rhaegal, and they come x2 in 1 core, so you could get a full playset of them with 2 cores.

How many decks do most players keep ready? I would like to have 6, 1 for each house. Do players proxy cards such as Valar Morgalis?

I feel like I have decks, half decks, and "idea piles" sitting all over the place. One of the reasons for buying playsets of all of the cards is so that you don't have to spend so much time making proxies and tearing decks apart when deck-building.

Even with access to 3 cores, if you build multiple decks out of the same house (and I currently have 4 Targ decks built), you need to proxy a considerable amount.

I used to try and keep 6 decks built, but I find it much more reasonable (aka many less proxies) to keep 3 decks built from 3 different houses.

Core eddard!

Can confirm. Have two in deck.

_{I'm serious. <.<}

Does anyone ever get 4 core sets for the plot cards for multiple decks, or is 3 the most?

At this point, I prefer to move the cards from deck to deck.

Does anyone ever get 4 core sets for the plot cards for multiple decks, or is 3 the most?

It all depends on how nutty you want to get. I'm guessing 4 isn't even the record.