
Jaime Lannister, also known as the Kingslayer, is a knight who has vowed as the lord commander of the Kings guard--in order to protect king from any harm.
In the series, it is shown that Jaime and his twin sister Cersie had affairs going on between them. Although his sister is already married to the King he and his sister's affair hadn't stopped. It is somewhere said by Tyrion, his younger brother, about the two siblings affair.