One of the long-lived children on Miri's planet, only slightly older than Miri, who reached puberty, contracted the fatal disease and died after attacking Captain Kirk in 2266.
Louise appears in the episode Miri. She is one of the Onlies, the children left alive after an experiment in prolonging life killed every adult on their world three centuries earlier. The same virus that slows their ageing turns on them as they enter puberty.
Miri tells the landing party that Louise was a little older than herself and had gone missing. She soon appears, disfigured by blue blotches and raving, and hurls herself at Kirk. He stuns her with a phaser set low, yet she dies anyway.
McCoy finds that the disease and not the weapon killed her, and that her body had aged a lifetime in days. Her fate shows Miri what awaits her and the landing party how little time they have.