Lydda Jath of Kathoon is like one of those rich kids who wants to be a singer, so her parents buy her a record deal and bribe radio stations to get them to play her songs, only in her case, she wanted to be a member of the Legion, and got her scientist father to give her powers, thus becoming Night Girl:
Unfortunately for her, while her father doted on her enough to give in to her wishes, and succeeded in giving her super-strength, her powers only worked at night or in deep shadows.
Which was fine on Kathoon, a world that exists in perpetual darkness, but presents a problem when attempting to engage in super-heroics in any well-lit areas.
Naturally, given this drawback, the Legion rejected her application.
However, she and several other Legion rejects went on to form the Legion of Substitute Heroes. It's kind of like if all of the losers from American Idol went on to form a band.
If the Legion was otherwise unavailable, the Subs would step in and do their best to fill in for them. Sadly, their best really wasn't all that great.
Still, it wasn't all bad news for Lydda, who was actually one of the more competent Subs. After all, the whole reason she wanted to join the Legion in the first place was because she had a crush on Cosmic Boy, and while she never managed to become a full-fledged Legionnaire, she did at least manage to catch Cosmic Boy's eye - and with that beehive hairdo, how could she not stand out? - and managed to get into his...er...corset.
Seriously; in the 70s, Cosmic Boy wore a corset. In fact, for the purposes of this picture, I decided to draw her actually wearing his corset costume. Honestly, remove the jacket, and that's Cosmic Boy's costume.
I like to imagine that as a surprise for her boyfriend, Lydda ordered something trashy from Fredrick's of Talok VIII, and when Cos stopped by the Subs' clubhouse to visit and she walked out wearing it and said, "Look what I bought for you!" he totally misunderstood her intentions...