1. Jeyne Westerling
Robb Stark marries a girl of his own choosing in both the books and TV series, but the girl is different depending on the medium. In A Storm of Swords, Robb and his army attack the Crag, the ancestral home of the Westerlings, who are allied with House Lannister. After he is injured by an arrow, he is nursed back to health by Jeyne Westerling, daughter of Lord Gawen Westerling. While at the Crag, Robb learns of Bran and Rickon's apparent deaths at the hands of Theon Grejoy at Winterfell. Distressed, Robb sleeps with Jeyne for comfort, and because he is an honorable man, he marries her afterwards. Like the TV show, this puts the kibosh on the previous arrangement Robb had of marrying one of Walder Frey's daughters. Fortunately for her, Jeyne wasn't at the Red Wedding where her husband, mother-in-law, and many of her husband's men were gruesomely killed. She was left at Riverrun because Robb didn't want to piss off Walder Frey anymore than he had to, something that TV Robb didn't think was necessary. Anyway, Jeyne is also pardoned by the Iron Throne, although this doesn't do her much good. Taken back to the Crag, she can't mary anybody for two years, and Jamie gives orders to kill her if any escape attempt is made. Her mother has also been dealing with Tywin Lannister and giving her daughter disguised potions to make sure that if Robb did impregnate her before he died, the baby won't survive. So yeah, this girl's life isn't exactly the ideal one she was expecting. For the TV show, Jeyne was replaced by the original character Talisa Maegyr. At first, some viewers who had read the books were confused because although her name was different, she was nursing wounded soldiers like Jeyne had. It was later revealed that Talisa was an official battlefield nurse rather than just tending to soldiers in her own castle. The circumstances in which she and Robb fall in love are different than the book as well. Rather than coming together in a grief-ridden one-night stand, Robb and Talisa fall in love through the traditional routes: flirtation, sharing personal information, and THEN having sex. Basically, he deliberately pushes himself into a corner in the TV show rather than taking said actions out of honor. Finally, Talisa's fate is significantly worse compared to Jeyne's. After recently telling Robb that she is pregnant, she is stabbed in the stomach multiple times and dies after bleeding out. I won't lie, I've been able to tolerate a lot of the gruesome deaths the TV show has thrown our way, but that one was especially hard to sit through. As for why the change happened, it's unclear. Show-runners David Benioff and D.B. Weiss wanted to give Robb Stark more to do in the TV show since his exploits in the books occur either off-screen or in the presence of third-party characters. Because of this, there were several plot changes so that we could see what Robb was up to during the conflict (this led to many character changes for Robb himself in the show, but that's for another discussion). What doesn't make sense is why the writers felt the need to start changing Jeyne's character into someone completely new. Even author George R.R. Martin wasn't comfortable with these changes, telling Benioff and Weiss that if they were going to alter Jeyne's character this much, they might as well just give her another name and make her into someone new. When asked about the change, the two writers go back to the "wanting to show what Robb was doing" excuse. Look, I get that you wanted to give Robb more screen-time, but to change his wife altogether? It was completely unnecessary. Plot-wise, it didn't add anything new other than emotional turmoil at seeing her killed at the Red Wedding, but we didn't need to see that because Robb and Catelyn were going to be killed anyway, and I guarantee that the viewers care more about those two than Talisa.