H is for... Hydroflax

Ruthless warrior monarch, known as the Butcher of the Bone Meadows, who reputedly ate his enemies, dead or alive. His constant warring had eventually led to so many injuries that he now existed merely as a head attached to a robot body. An explosion caused the fabled Halassi Androvar diamond to become lodged in his cranium, and this is why River Song got married to him. She planned to have a doctor remove the jewel so that she could steal it and sell it to a group known as the Shoal of the Winter Harmony, who regarded it as a sacred totem. Her assistant, Nardole, accidentally fetched the Doctor instead of the medic she had originally contacted. River did not recognise him in his twelfth incarnation. On stealing Hydroflax's whole head when the deception was revealed, the robot body went after River and the Doctor, as it could operate independently. However, it took for itself two new heads - that of Nardole and that of River's other husband, Ramon.
The TARDIS was unable to dematerialise when someone was both inside and outside the ship at the same time, and the robot was able to force it's way on board when River thought that it was Ramon who was at the doors.

The handover of the diamond was to take place on the Harmony and Redemption, a luxury space-liner frequented by rich criminals. It transpired that the Shoal idolised Hydroflax even more than the diamond as he had conquered them, so would kill anyone harmed him. When the robot arrived at the exchange, it decided it had no further need for the King's head, and disintegrated it, revealing the gem. The robot body was temporarily disabled when it was overloaded with financial information.
The spaceship crashed on the planet of Darillium, where the Doctor gave the diamond to a man who was helping clear away the debris. He was advised to build a restaurant on the site, overlooking the famous Singing Towers of Darillium. The Doctor met River there for their final date, before her ill-fated trip to the Library planet. The Hydroflax robot now worked at the restaurant, still with the interchangeable heads of Nardole and Ramon.
Soon after he had left River, the Doctor extracted Nardole's head from the Hydroflax robot body and attached it to a new one.

Played by: Greg Davies. Appearances: The Husbands of River Song (2015).

• Davies is best known for stand-up comedy, or comedic drama roles in series such as Cuckoo.