McCoy's former sweetheart, whose identity was taken over by the salt-craving shapeshifter of planet M-113 after it killed her.
Nancy Crater was once romantically involved with Leonard McCoy and later married the archaeologist Robert Crater, joining him on planet M-113. When the Enterprise arrives for a medical check, the woman who greets them is not Nancy at all but the last surviving member of a native species that drains salt from its victims and can appear as whoever it chooses.
The creature killed the real Nancy and uses her image, which each observer sees differently, to approach the crew. After several deaths, McCoy is forced to fire on the being he still sees as Nancy, killing it.