He is often referred to as "Lame Lothar" due to the pronounced limp he has in his left leg.
Lothar is a son of Walder Frey, the Lord of the Crossing. He is the steward of the Twins, managing the day-to-day running of the castle. He often plans and organizes the fine details to achieve his father's goals.
Lothar travels to Riverrun with Lord Walder's bastard son, Black Walder Rivers, and meets with King Robb Stark and his advisors to discuss the reforging of the Stark-Frey alliance for Robb's planned attack on Casterly Rock. The Freys carry Lord Walder's demands, which includes a formal apology from Robb, the castle of Harrenhal and all of its lands and incomes, and for Edmure Tully to marry Roslin, one of his daughters. Edmure is reluctant to marry a woman he has never met, but is eventually convinced to go through with the arrangement
Lothar hires a group of assassins to pose as musicians at Edmure and Roslin's wedding. Talisa Stark comments on their talent, and Lothar replies that they should want to be good due to their cost. After the bedding ceremony, Lord Walder rises to make a toast to Robb and offer a marriage gift to his Queen, signaling Lothar to draw a knife from his sleeve and repeatedly stab the pregnant Talisa in the stomach, killing her and her unborn child. He presumably participates in the ensuing massacre.
One of Walder Frey's many sons, Lothar accompanied his brother Black Walder to Riverrun where they met with Robb Stark to negociated the marriage of Edmure Tully to their sister Roslin. On the night of the wedding, Lothar stabbed Robb's pregnant wife Talisa inthe abdomen, thus initiating the massacre of the Red Wedding. Arya Stark, using skills learned from her time in Braavos, killed him and served him in pie to his father, Walder Frey.