Ser Kevan Lannister is a knight from House Lannister and is the younger brother of Lord Tywin Lannister. Following the War of the Five Kings he was named Lord Paramount of the Westerlands, seeing as the main branch of House Lannister had been extinguished.
Kevan alongside his brothers, fought in the War of the Ninepenny Kings and won his knighthood. Upon the return to Casterly Rock, Tywin charged Kevan with the task of ridding the westerlands of robber knights and bandit outlaws. Tywin then had Kevan collect the outstanding debts owed to Casterly Rock, and those who could not pay immediately had to surrender a family member as a hostage till the debt was settled. As Ser Harys Swyft not able to pay at the time, he handed his daughter Dorna into Kevan's custody.
Kevan soon fell in love with Dorna and married her. They would later have four children: Lancel, twin boys Martyn and Willem, and a newborn daughter, Janei.
War of the Five Kings
Kevan Lannister fought alongside his brother from the outset, leading the center in the Battle of the Green Fork, and commanding the force that draws out Lord Randyll Tarly before the Battle of the God's Eye. While he is present in the Battle for King's Landing, he commands their left flank, and upon hearing of the death of his brother, surrenders.
Renly Baratheon, knowing of Kevan Lannister's skill and renown - both for his ability and his loyalty - decides to spare him, and name him the new Lord Paramount of the West, in place of his nephews. Now-lord Kevan returns to the West, setting about restoring the ruins of his homeland.