Elizabeth X

Elizabeth X of the United Kingdom, (also known as Liz Ten) was the Queen Regnant of the United Kingdom of Great Britain and Northern Ireland when Earth was devastated by solar flares. She'd grown up on stories of the Doctor, as had her whole family.

She found a Star Whale and enslaved it (or so she thought she had; in actuality it volunteered) to form the base of Starship UK. She then wiped her mind so that she would not have to deal with the consequences. Her government slowed down her body clock, keeping her effectively immortal. She thought she was 50 and had been on the throne for ten years, not knowing she always lived the same ten years, always found out the truth about Starship UK at the end of it, and always wiped her mind once more. She also placed this system throughout the ship and everyone would always choose to forget. Those who didn't were sent below, to fuel the Star Whale. She feared that if the Star Whale was freed that it would then fly off, killing all who lived on the ship. She wore a mask to disguise herself from the public whilst investigating.

300 years later, when The Doctor and Amy Pond came to Starship UK, they found out the truth and told Elizabeth. Amy freed the Star Whale but the beast continued to carry Starship UK as it was old, kind and the last of its species, just like the Doctor. Liz then was able to live a happy life along with her kingdom. 

In the 52nd century, she was still the Queen but showed signs of age by the fact that her hair was greying. By this time she was also guardian of The Royal Collection. She caught River Song in 5145 attempting to steal a Van Gogh painting of the Tardis exploding. After holding River at gunpoint she saw the painting and learned of whom it was meant for before letting her go. (DW: The Pandorica Opens)

Liz X could be "Your Majesty", mentioned in "The Big Bang", as the Orient Express was said to be "in space" (possibly akin to the Titanic in "Voyage of the Damned")