Ser Robar Royce is a knight and highly successful at jousting tournaments.
He joins Renly's Kingsguard, because he is not in line to inherit and the glory of tournaments is not going to give him a future. Catelyn estimates his age as being in his early twenties. Ser Robar is guarding Renly's tent and comes in when he hears Brienne call out. Catelyn Stark persuades him that Brienne is innocent and he holds off the others, allowing them to escape.
Ser Loras Tyrell murders Ser Robar in a fit of rage for his failure to protect Renly and kill Brienne. Later, Loras regrets this, as Robar was his friend and he comes to believe that Stannis is responsible for Renly's death.