Terrible Zodin

The Terrible Zodin was encountered by and reminisced about the Doctor on a number of occasions. He first met her some time prior to his second incarnation. Iris Wildthyme also claimed to have met her.

Adventures
Zodin was involved in an adventure which caused the Doctor to interact with multiple incarnations of himself. Following this she erased their memories of the incident using "mind rubbers", preventing the later Doctors involved from remembering having experienced the events before (MA: Cold Fusion).

This did not prevent the Doctor from remembering enough of the adventure to frequently bore people to sleep with a long-winded account of it, although he was incapable of consistently recalling whether she was assisted in her schemes by mutant kangaroos or by giant grasshoppers (NA: Legacy).

Jamie (PDA: The Colony of Lies) and Mel both experienced the incident (MA: Millennial Rites).

When the Doctor's second incarnation visited a UNIT reunion in the early-mid 1980s, he described Zodin to Brigadier Lethbridge-Stewart, and indicated the Brigadier would have his own encounter with the Zodin alongside the Doctor in his personal future. (DW: The Five Doctors). (The Brigadier however may have assisted Iris Wildthyme against Zodin in a separate adventure on the planet Mars.)

The Third Doctor disputes that ever took place (PDA: Verdigris) although concedes that accounts of his own conflict with Zodin may not be entirely accurate when confronted with the implausibility of her motives and of grasshoppers jumping eighteen-storey buildings (NA: Legacy).

The Tenth Doctor and Martha Jones at some point met the Terrible Zodin. (Martha Jones Blog July 1)

Sarah Jane Smith had an unspecified encounter with a species named the Zodin, but they may not be related. (SJA: The Mad Woman in the Attic)

Personality
The Seventh Doctor once described Zodin as a celebrated sword-swallower at the Grand Festival of Zymymys Midamor and by no means any kind of actual villain, who did an impressive trick with a scimitar (NA: Lungbarrow), although the Doctor did consider her to have ego-maniacal and megalomaniac tendencies (MA: Millennial Rites). She possessed claws, or talons (PDA: The Colony of Lies). The Doctor considered her a villain of rare guile and cunning, he stated that they didn't make villians like her anymore. At one point his unstable memory caused him to call Peri, Zodin. (DW: Attack of the Cybermen).

Behind the scenes

 * The first reference to the Terrible Zodin occurs in a scene in The Five Doctors when the Second Doctor mentions her to the Brigadier. Because several variant cuts of The Five Doctors exist and because Patrick Troughton apparently elaborated on the script, the amount of details on her and the details themselves vary from version to version.
 * The Terrible Zodin has become something of a running joke in Doctor Who in that she is always discussed, but so far has never actually appeared on screen or in print.