Garrett Flowers is a bastard of House Tyrell, and the first cousin to Mace Tyrell, the current Lord of Highgarden. He is the youngest son of Garth Tyrell, and he has an older brother, Garse Flowers.
A Feast for Crows
When Garrett's father, Garth Tyrell, travels to Oldtown to take ship to King's Landing to serve on the small council as the new master of coin, Garrett and his brother Garse accompany him, as Lord Tywin Lannister had promised to find a place for them. When informing Queen Regent Cersei Lannister about this, Lord Mace Tyrell suggests Garrett and Garse be named to the City Watch of King's Landing. However, Cersei, wishing to prevent the presence of another Tyrell on the small council, prevents Garth's promotion by quickly naming Lord Gyles Rosby as the new master of coin.