She's a structural device that highlights the Starks' tragic tendency toward honorable self-sabotage, honestly. Robb's marriage to Talisa/Jeyne isn't really about her character—it's about his choice. He breaks a vow for love or honor, and the narrative uses that wife character as the catalyst for the Red Wedding. She's the personification of Robb's youth, his idealism, and ultimately his fatal error. You could replace her with a dozen different personalities, and the plot function would be the same: she’s the reason Walder Frey feels betrayed.
That said, the show's version, Talisa, gives her more direct agency. She’s a battlefield medic, foreign, vocal. She challenges Robb. But even then, her primary role in the Stark saga is to be the beautiful, pregnant reason everything falls apart. Her murder is the final, brutal punctuation on the Stark downfall in Season 3. In the books, Jeyne Westerling is a quieter presence, almost a pawn of her mother, and her role feels more like a geopolitical misstep than a romance. It’s less about who she is and more about what she represents: a lost alliance, a broken promise, the cost of ruling with heart over head.
The lingering impact is on Catelyn, mostly. That poor woman sees her son’s kingdom and life unravel over this marriage. So the wife’s role ripples out—she’s the stone that starts the avalanche that buries the King in the North and his mother. She’s a plot grenade with a pin pulled by Robb himself.