Ever since the Red Wedding, book readers have speculated about the fantastical character Lady Stoneheart, a resurrected, bloodthirsty version of Catelyn Stark from Martin's novels. Beric Dondarrion -- whom we have seen die and return to life on the show -- sacrifices himself to yank Catelyn back from the other side, and Lady Stoneheart, as she comes to be called, leads the Brotherhood Without Banners in his place. Only hers are more malicious, and focused on people who've done her wrong (the Freys, the Lannisters, etc.).
The character's worth bringing up because it appears that Sansa may essentially be taking Lady Stoneheart's place in the show's streamlined narrative. With the fiery makeover and a warrior's vitality that erupted out of her battle wounds, Sansa doesn't arrive at Castle Black just content to cozy up in the corner; she rallies Jon, rallies the troops, and plans a march back on Winterfell. When Littlefinger sneaks back on to her radar, offering Sansa the Vale's troops, she both takes advantage of and eviscerates her casual oppressor. What happened with Ramsay haunts Sansa, and in her big scene, she sharpens the pain into a spear to plunge into Littlefinger's heart.