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 project. When a landing party from the USS Enterprise encountered her in 2266, she was over three hundred years old.
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 the 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 project, 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 the rest of the surviving onlies would eventually develop the fatal disease when they reach puberty, and before then, starvation would likely kill the younger onlies. Miri was starting to develop the disease on one of her arms, but she was cured of the infection by a vaccine developed by Doctor McCoy.
When the Enterprise left their world, Miri and the other Onlies were left in the care of a team of medical specialists from the Enterprise, with teachers, advisors, and (hopefully, according to McCoy) truant officers en route. (TOS: "Miri")
Miri was played by actress Kim Darby.
In the first draft of the "Miri" teleplay, Miri had more of a relationship with Jahn than is depicted on screen. In their distorted reality, Miri was similar to Wendy from Peter Pan, while Jahn represented the titular character from that story. Also, when the Onlies learned she was approaching the end of her childhood, Miri was denounced, by them individually, as an unsuitable playmate. (The Star Trek Compendium, 4th ed., p. 42)
In the final draft script of "Miri", the character of Miri was initially described as "a young girl" with a "pretty face" and "lovely eyes." The script also regarded her as having hidden "maternal instincts." It was also established that she used to play with a boy creature that the Enterprise landing party encounters prior to meeting Miri, and that the house where she is found by the landing party used to be her home (an abode which was described, in the scripted stage directions, as "middle class"). Even in the final draft script, there was more between Miri and Jahn than in the final version of the episode, with the teleplay stating about them, "There is something between these two – 300 years ago, if everything hadn't gone wrong, they would have been childhood sweethearts, we feel. Somewhere deep inside of them, they know this, too."
In the novel The Cry of the Onlies, Miri was still on the Onlies' homeworld when Jahn rebelled against the Federation educators left there to help the Onlies into adulthood; she and several other Onlies were killed as a result of Jahn starting a fight by grabbing a security guard's phaser. Kirk is devastated by the news, so much that Spock mind melds with him to temporarily suppress his memories of his role in Rayna's "death" before the Enterprise rendezvous with Flint, calculating that two such traumatic memories would be more than Kirk could handle at once.
The William Shatner novel Preserver featured a return to this planet, wherein it was revealed the planet had been duplicated from Earth on a subatomic level by the Preservers. However, Forgotten History states that various subspace anomalies indicated the planet was from an alternate timeline.