The defiant leader of the surviving children on Miri's world, who leads them against the adult strangers they call grups.
Jahn is the oldest boy among the Onlies, the children who survived a life-prolonging experiment that killed every adult on an Earth-like planet three centuries before Miri. The children age only one month for every hundred years, but when they reach puberty the disease strikes and kills them.
In Miri, Jahn distrusts Kirk's landing party and leads the children in stealing their communicators, cutting them off from the Enterprise while McCoy races to find a cure. He also takes Janice Rand captive. Kirk eventually persuades the children that the grups are trying to save them. Jahn was played by Michael J. Pollard.