Warning: major spoilers ahead for the second episode of Game of Thrones Season 7, "Stormborn."
Despite six seasons of battles, massacres, and assassinations, Game of Thrones still consists of a large cast of characters, and many of them still have murky motives. Now that all the major powers are getting together to fight one war or another, the true colors of characters are beginning to come out as never seen before. "Stormborn" gave us closer looks into the minds of Arya and Varys and even Melisandre. The biggest game-changer of a reveal, however, came with Theon Greyjoy's decision to abandon his sister and what was left of their fleet.
"Stormborn" ended on a truly remarkable sequence at sea as Yara and Theon were attacked by the ships of their uncle Euron, still on his mission to prove himself to Cersei. Euron had the greater numbers, the element of surprise, and fiery weapons that pretty much settled the battle as soon as it began. With the sails and decks of Yara's fleet burning, she and her soldiers were never going to win. Still, the Ironborn loyal to Yara fought their hardest, and they took a lot of Euron's forces down with them.
Unfortunately for Yara, the onslaught of Euron's forces was too much for Theon. All was basically lost by the end, with the Sand Snakes dead, Ellaria captured, and the fleet destroyed. Only Theon was left to save his sister after she was outfought by Euron. When faced with the opportunity to save his sister and/or kill his uncle, Theon chose a route that showed he is still the coward he was broken into by Ramsay Bolton. Theon literally jumped ship on his sister, leaping overboard with Yara in the clutches of the victorious Euron.
While Theon hasn't exactly even been the bravest or most admirable guy on Game of Thrones, his remorse over what he did when he took Winterfell combined with his intention to do better after he and Sansa escaped Ramsay pointed toward a redemption arc. He reunited with the one member of his family who didn't want to kill him or mock him for what Ramsay did to him, and they formed an alliance with a woman who happened to have three dragons, a Dothraki horde, an army of Unsullied, and a variety of Westerosi soldiers courtesy of Ellaria and Olenna Tyrell. The time had come for him to take a stand.
Alas for Yara, Theon simply couldn't do it. Deep down, he still seems to be the man who could so easily be manipulated. He has always attached himself to stronger people around him; when left alone with nobody to protect him or show him exactly what to do, he fled. Oh, Theon.
Admittedly, Theon is still deeply and understandably traumatized by what Ramsay did to him, and he likely would have gotten both Yara and himself killed if he'd tried to battle Euron. Still, Yara was clearly disappointed that he bailed. With Euron's triumph, Yara's capture, and Theon's abandonment, it seems that the Ironborn are already knocked out of the fight to win the Iron Throne for Dany. Only time will tell if they still have a part to play. As for Theon... well, he may have just run out of chances to redeem himself. We'll have to wait and see.