Giselle Smith of Triton (one of Saturn's moons), was caught in an invasion wave of destroyer units when the Legion of Super-Heroes appeared to defend the population. In the shelter of the ski resort she was visiting at the time, Giselle was beaten by bullies because of her rare mutation enabling her to control her own metabolism. Saved by Invisible Kid, she later helped the Legion defeat the destroyer units; though afterwards she claimed that she hated the United Planets and the Legion, and only fought to save herself.
Next time she appeared, she had become a member of the United Planets Young Heroes, the UP's answer to the Legion - under the code name Gazelle. When the team failed, Gazelle realized she had been wrong about the Legion, and applied for a Legionnaires try-out. Out of four candidates - Gazelle herself, Night Girl, Turtle and Sizzle, Giselle was the only one to be admitted into the Legion core team on a full-time basis.
Gazelle possess the ability to manipulate her metabolism in order to achieve several effects, but most prominently boosts it giving her a substantial degree of super strength and speed. But she is also capable of slowing down her metabolism to such an extent that she enters a state akin to hibernation.