Santa Claus

Santa Claus — also known as Father Christmas or, according to the Eleventh Doctor, "Jeff" or "Geoff" — legendarily distributed presents to good children on Christmas morning under their trees. He played an important part in the Christmas tradition on the planet Earth.

Profile
Santa maintained a workshop where he created toys. (TVC: A Christmas Story) In order to deliver presents to all children in a single night, Santa used time loops and clones of himself. (ST: The Man Who (Nearly) Killed Christmas)

Appearances
The First Doctor visited Santa Claus in his workshop and helped save Christmas. (TVC: A Christmas Story)

The Second Doctor warned Santa of a future where the toy workshop spanned over two thousand worlds and the fabric of space was being torn by his time looping. A solution was found involving a fleet of robot Santas and newly calculated, more stable wormholes. (ST: The Man Who (Nearly) Killed Christmas)

Santa helped Iris Wildthyme save Panda from Mary Christmas. (BFIW: The Claws of Santa)

Impersonation by the Doctor
The Fourth Doctor dressed as Santa and snuck into the Brigadier's house with Romana in order to leave him a Christmas present. (ST: Special Occasions: 3. Better Watch Out, Better Take Care)