Open-skinned

For Sontarans, to be open-skinned was the act of going without their helmets even when facing battle in their Sontaran space armour.

History
Commander Stark, flanked by two soldiers, appeared open-skinned as he represented the Sontarans in the Alliance which imprisoned the Eleventh Doctor in the Pandorica in 102. (TV: The Pandorica Opens)

Jingo Linx removed his helmet after arriving in 13th century England. (TV: The Time Warrior)

Group Marshal Stike and Major Varl appeared open-skinned in Seville in the 1980s. (TV: The Two Doctors)

His honour insulted by the Tenth Doctor, General Staal of the Tenth Sontaran Battle Fleet took his helmet off to look into the eyes of his enemy just prior to the Sontaran invasion of Earth in 2009. Upon returning to his Sontaran flagship, his subordinate Commander Skorr requested that he be allowed to share that honour, which was granted by Staal. (TV: The Sontaran Stratagem) Following the death of Skorr, Lieutenant Skree appeared open-skinned. (TV: The Poison Sky)

Jask appeared open-skinned as he fought Mickey Smith and Martha Jones on Earth in 2010. (TV: The End of Time)

A Sontaran resident of Trap Street, London in the 2010s was open-skinned. (TV: Face the Raven)

Commander Skarr and his subordinate appeared open-skinned as they attempted to infiltrate the town of Christmas during the Siege of Trenzalore. (TV: The Time of the Doctor)

Commander Strax, a disgraced nurse, regularly appeared open-skinned. He was seen as such as he served during the Battle of Zaruthstra in 4037, where he was summoned by the Eleventh Doctor to participate in the Battle of Demons Run. (TV: A Good Man Goes to War) After being seemingly killed in the battle, he was revived by Vastra and Jenny Flint, who he accompanied to the Victorian era, where he served as their butler. (WC: The Battle of Demons Run: Two Days Later) During this time, Strax wore human clothing though he did assume his Sontaran space armour when occasions called for it, such as when fighting the followers of Winifred Gillyflower. (TV: The Crimson Horror)

Field Major Styre removed his helmet after disembarking from his Sontaran scout ship on Earth in 16087. His commanding Marshal also appeared open-skinned. (TV: The Sontaran Experiment)

Undated events
Major Skar appeared open-skinned while leading soldiers against Lumpy the Dalek on Sontar. (GAME: The Doctor and the Dalek)

Commander Halke, a victim of the Minotaur, was open-skinned. (TV: The God Complex)

During the Sontaran invasion of Gallifrey, Commander Stor removed his helmet after entering the Doctor's TARDIS. (TV: The Invasion of Time)

Suicide Specialists appeared open-skinned. Uniquely, Sontarans loyal to appeared wore transparent helmets which revealed their faces, adorned with beards which resembled their Master. (COMIC: The Judas Goatee)

In an alternate timeline where Sontar was invaded by the Cybermen, Sontaran-Prime appeared open-skinned. (COMIC: Supremacy of the Cybermen)

Behind the scenes

 * Group Marshal Nathan and Turner appear open-skinned in NOTVALID: A Fix with Sontarans.
 * Sontaran variants appeared both with helmets and open-skinned in the video game Legacy.