An aristocrat was a member of the highest social class in a society.
Lord Kenilworth was an English aristocrat who led an expedition to Egypt in the late 19th century. Guided by the Fifth Doctor, he found the tomb of Nephthys. He took artefacts from the tomb back to England, including a mummy which was in actuality Nyssa. When the identity of the mummy was revealed at a mummy-unwrapping party at Kenilworth Hall in 1896, the Doctor determined it would take a century for Nyssa to completely revive. Kenilworth promised to keep her safe until then. (PROSE: The Sands of Time)
Lady Christina de Souza was a minor British aristocrat and a thief. After their exploits on San Helios, she asked to join the Tenth Doctor in his travels. He refused, still recovering from the loss of Donna Noble. (TV: Planet of the Dead) When she met him again in his eleventh incarnation, he asked her to join him. By then, she had obtained the technology to travel in space from Henry van Statten's Vault. She turned him down his offer, preferring to explore on her own. (COMIC: The Eye of Ashaya, TV: Dalek)
Matilda was an aristocrat who painted a nude study of the Eleventh Doctor. When King Charles II came looking for him, he hid beneath her skirts. The king discovered the Doctor and imprisoned him in the Tower of London. (TV: The Impossible Astronaut)