Teran Lannister is a major character in the fifth and sixth seasons. He was previously a recurring character in the second, third, and fourth seasons. He is the firstborn son of Tybalt and Cynda Lannister, and the older brother of Ser Alton Lannister.
Teran Lannister was originally the heir to Lannisport, but was disinherited after he supposedly turned traitor and allied himself with Robb Stark after being captured by him. In truth, he was secretly working for Roose Bolton, who was planning to betray Robb himself, but after refusing secret orders from Lord Bolton to participate in the Red Wedding, Teran turned on him and originally intended to join the Night's Watch. This plan never came to fruition, as he ended up joining Grayburn's Legion as one of Carmine Grayburn's advisers.