An exact double of Supergirl and a jealous Kandorian scientist who constructs a device that sap's Supergirl's powers so that she might take her place (Act No. 279/2, Aug 1961: "Supergirl's Secret Enemy!").
Lesla-Lar's diabolical switch is discovered in time (by Krypto the Superdog) and she is ultimately sent back to Kandor (Act No. 282/2, Nov 1961: "The Supergirl of Tomorrow!"). Later, she foolishly allies with three Phantom Zone criminals who turn on her and use a powerful Kryptonian weapon to vaporize her (Act No. 297/2, Feb 1963: "The Forbidden Weapons of Krypton!").
The psychic energy of Lesla-Lar apparently survives as, years later, she attempts to inhabit Supergirl's mind (SF No. 206/1, Mar/Apr 1981: "Strangers at the Heart's Core!").