Shilard Fitz-Oesterlen is a Spy. You play him on your opponent’s side of the board, where his 7 strength counts toward their Close Combat row, and in return you draw 2 cards. Since cards are your main resource across three rounds, that trade is often worth it.
There is no fixed seller for this one. It turns up as a random reward when you beat ordinary gwent players such as merchants, innkeeps and villagers, so keep challenging everyone you meet. It counts toward Collect ‘Em All.
