A girl on a planet where a failed life-extension project killed every adult, leaving children who aged only very slowly.
Miri is one of the Onlies, children who survived on an Earth-like planet after an experiment to prolong life created a disease that killed all the adults. The children age extremely slowly but die horribly when they reach puberty, and Miri is approaching that age when Kirk and his landing party arrive and become infected themselves.
She develops a crush on Kirk and, jealous of Janice Rand, briefly helps the other children capture her. Kirk wins her back, and she helps him reach the children while McCoy develops a cure. She was played by Kim Darby.