Staal

General Staal "the Undefeated" of the Tenth Sontaran Battle Fleet was a Sontaran general during the Sontaran invasion of Earth in 2009. His nickname of "Undefeated" was remarked by the Tenth Doctor as not a very good one as it could not remain if he was defeated and jokingly suggested "Staal the not-quite-so-undefeated-anymore-but-never-mind" as a replacement.

He had a great love of warfare and regretted his race did not fight in the Last Great Time War, which he considered the universe's greatest war. He forged an alliance with Luke Rattigan to aid the Sontaran conquest of Earth. He had an ambition to personally kill the Tenth Doctor on meeting him. (TV: The Sontaran Stratagem)

Staal remained in the Sontaran flagship when he ordered his subordinate Skorr to attack UNIT. He was contacted by the Doctor shortly after claiming the Doctor's TARDIS as his trophy. After his plan of turning Earth into a cloning planet for more troops failed, he furiously decided to destroy Earth by force.

Rejecting the Doctor's offer to let him leave Earth alone or be incinerated, Staal mocked him. He and all but one of his troops were then killed by Luke, who gave his life to destroy the flagship. (TV: The Poison Sky)

Personality
Staal possessed a personality that was typical for a Sontaran. He loved warfare and was unafraid of death, even mocking the Doctor when the Time Lord threatened to blow up his ship. Staal was also manipulative and treacherous, having manipulated Luke Rattigan into helping him by saying that he would take him and some other teenagers to another planet to rebuild the human race, when he was actually planning to use him as target practice when he was no longer needed. Staal showed no respect for humanity, claiming that "we say the same of humans" when Luke Rattigan asked how Sontarans told each other apart and called them "stinking human beasts". He was stunned when Luke gave his life to stop them, his last expression being one of incredulity.

As with other Sontarans, Staal had a strong sense of honour; he apologised to Privates Harris and Gray for paralysing their legs and conditioning them rather than killing them, saying that death would have been more honourable for them, and took extreme offence when the Doctor accused him of being a coward for disabling Earth guns and using poison gas when, as the Doctor admitted, they could blast Earth out of the sky. He also took offence when the Doctor remarked on what he thought wasn't a great nickname, asking what would happen if Staal were to be defeated. (TV: The Sontaran Stratagem, The Poison Sky)

Behind the scenes

 * General Staal and his invasion of modern London is mentioned by Commander Strax in the fourth-wall-breaking cinema intro for The Day of the Doctor. He is disappointed to learn that the anniversary feature is about the Doctor as opposed to a Sontaran historical drama featuring heroic accounts of their various military campaigns which he was anticipating with his clone batch.