Strax

Commander Strax was a Sontaran commander who was demoted to nurse as punishment for an unspecified failure.

Biography
He was one of many people who owed the Doctor a debt. In the year 4037, he was tending to human soldiers at the Battle of Zaruthstra when he was summoned by the Doctor to help in the mission to rescue Amy and her baby at Demon's Run, which offered him a chance to once again experiance the pleasure of battle. He was mortally wounded in the battle with the Headless monks when one damaged his probic vent. In his final moments, Strax noted how he had once longed to die in combat (which is common among Sontarans), and now, he didn't find it as enjoyable as he thought. Rory tried to comfort him, saying he was sure he would recover, but Strax wasn't fooled, reminding Rory that he was a nurse, too. However, he admitted that he had had a good life, stating that he was nearly 12 years old, implying Sontarans live within the span of 10-15 years due to their dedication to a life of warware. (DW: A Good Man Goes to War)

Personality
For a Sontaran, Strax was very emotional and had an amiable relationship with the others in the Doctor's rescue party. He showed an affection for children, saying to one boy he healed that he should become a great warrior (also expressing hope that they would meet as enemies on the field of battle) and also offering to nurse Amy's baby - he gene-spliced himself to produce "magnificent quantities of lactic fluid."