Miri was a young woman from a planet originally almost identical to 1960s Earth. Like all survivors on her world, Miri was infected with the life prolongation complex. When the USS Enterprise landing party encountered her in 2266, she was over three hundred years.
The landing party discovered Miri hiding in an abandoned building. When first encountered, she was terrified of these "grups", and begged them not to hurt her. Eventually, they won her trust. From her, they learned that grups had become insane, and the children, or Onlies, learned to fear them and hide from them.
The landing party had become infected with the life prolongation complex, and could not return to the ship. Miri helped them find the building where scientific research had been conducted, and performed various small tasks for them, mostly because she had developed a crush on Captain Kirk.
Ultimately, Miri became jealous when she saw Kirk embracing Janice Rand, who was crying in fear of the disease. Miri took this the wrong way and arranged with the other Onlies to kidnap Rand. Kirk managed to convince Miri to take him to the children, by explaining to her that she and they would eventually develop the fatal form of the life prolongation complex, and that long before then, starvation would likely kill the younger children. She was cured of her infection by a vaccine developed by Dr. McCoy.