Rok Ma was a Judoon in search of the Invisible Assassin. When he first met the Doctor his team had taken control of Elvis the King Spaceport. He assisted the Doctor and Nikki Jupiter in bringing down the Criminal Underworld of New Memphis. After the investigation finished, he stayed with Nikki and joined her Detective Agency. (NSA: Judgement of the Judoon)
Personality
Rok Ma was a typical Judoon when he first met the Doctor, but as he spent more time with him he became more understanding of the human mind and his personality changed to become more happy and less fixated on Judoon justice.
Rok Ma was the first Judoon known to have made a joke and laughed, this even shocked the Doctor when he realised. (NSA: Judgement of the Judoon)