Night Girl is Lydda Jath, a member of the Legion of Super-Heroes in the 30th Century. Born on the sunless planet Kathoon, her scientist father gave her the power of super-strength. Unfortunately, this ability only works when she's not in sunlight. The Legion originally rejected her, and she became a founding member of the Legion of Substitute Heroes. Her romantic partner and later husband is Cosmic Boy.
In Reboot Legion continuity, Night Girl is only seen as a member of the substitutes. In Prime Legion continuity, Night Girl is a member of the Legion Reserves. Night Girl was created by Edmond Hamilton and John Forte, first appearing in Adventure Comics #306. (1963)