Luke Sullieman

 was a Therian student of River Song who became a good ally of hers while she taught on Chasca Major.

As a student
Luke was a student of archaeology at the university of Chasca Major, which River came to particularly like. Wanting to teach him the "fun parts", River convinced Luke to break with her in the museum and open a tomb, which was revealed to be the sleeping place of the "Decayed" Master. Luke then accompanied River and the Master on Chasca Minor, where the Time Lord wanted to retrieve his TARDIS. When he was bitten by one of the Therians, his own Therian nature was awoken and turned him into a savage feline form, but River was able to bring him back to his original form. (AUDIO: Animal Instinct)

Luke eventually graduated with an A, an evaluation "too generous" according to River. (AUDIO: Carnival of Angels)

Investigating Angels
Luke accompanied River in New York City, during the 1930s, when she went there to investigate the Weeping Angels. When Marty Stone came to them, he went investigating the coin he had and met Dean Cray, who recognised in him a "fellow" freak, and his employer, Miss Quirke, who invited him to join their freak circus. Luke refused. Later on, he proved instrumental in convincing Dean to betray Miss Quirke and help him and River to defeat the Angels, who had taken possession of a carnival in Coney Island. After that, River sent Luke back home together with Dean, while she remained there to face the Angels on her own. (AUDIO: Carnival of Angels)