Louise was a young woman from a planet originally almost identical to 1960s Earth. Like all survivors on her world, Louise was infected with a virus. When the USS Enterprise landing party encountered her in 2266, she was over three hundred years old.
That same year, Louise entered puberty, and was completely transformed by the infection. Her fellow children ran away from her screaming, then she attacked Captain James T. Kirk. She died after Kirk shot her with a phaser set on stun; this apparently proved too much for her in her infected and weakened condition.
Louise had been only a little bit older than Miri was at the time of her death. (TOS: "Miri")
Louise was played by Irene Sale.
Louise's exact age, in comparison to Miri, is left unclear in the episode. In the final draft script of "Miri", however, Louise was described as being slightly older than Miri. This line of dialogue was drastically changed on-screen, as Miri instead says, "She was just a little bit older than I am... when it happened."