Lesla-Lar was one of the many Kryptonians living in the bottle city of Kandor, located in Superman's Fortress of Solitude. A brilliant scientist, she soon grew to be jealous of Supergirl's heroics on Earth and began harboring criminal aspirations. She would build a device that stripped Supergirl of her powers via a Kryptonite ray. She would use this device on the very day that Superman decided it was time to reveal Supergirl to the world. Unaware of Lesla-Lar's involvement, Supergirl would retire to her Linda Lee identity. Lesla-Lar's involvement in this would eventually lead to Supergirl being adopted by Fred and Edna Danvers, who would raise Supergirl for the next few years and later share her secret identity. 
The next phase of her plan involved taking Supergirl's place and starting criminal operations on Earth. Building another device, she would use it to switch places with Supergirl. Transporting Supergirl to Earth, and mentally conditioning her to believe she was Lesla-Lar and continue her every day activities, while Lar would take her place on Earth. 
On Earth, like any Kryptonian, Lesla-Lar would gain super-powers akin to Superman and others. Right after arriving on Earth, she would make contact with Lex Luthor, who was in prison at the time. Stealing him away and replacing him with a dummy, the two would succeed in stealing a large sum of money before Lesla-Lar put Luthor back in his cell to avoid any suspicion. The two would agree to work together in the future. However, Lesla-Lar would be forced to return to Kandor when Supergirl (still thinking she was Lesla) witnessed the evil "Supergirls" activities on Earth and was about to report her. Whenever Lesla-Lar and Supergirl switched places, Supergirl would have vague, if any memories of the experience.
Realizing the best way to operate as criminals would be to have a "good" Supergirl operating in the public eye, while secretly aiding Luthor in his criminal endeavors, Lesla-Lar began her next plot. Switching places with Supergirl once more, she would ask Luthor to come up with a way to make a Kryptonite Ray that they could use against Superman. She would come up with a "cure" for her powerlessness, and aid Superman in saving endangered people at the site of a canyon earthquake. Convincing Superman that she should be revealed to the public, the two go back to the Fortress of Solitude as Superman needed to check on an experiment before the revelation. However, there Krypto would discover that the Supergirl in their presence is not the real Supergirl. While Superman was otherwise occupied (and thus unaware) Krypto would learn the truth, and use a switching ray to make both girls return to their rightful places. Supergirl would have no memory of the time she spent on Kandor, but her powers were still gone. 
Lesla-Lar's attempts to return to Earth were halted when Supergirl went trips to the past and future, where she learned that her powers were only effected in the present. Deciding to stay on Earth and give up her Supergirl identity and live as a normal girl. However, before she could dispose of her Supergirl costume she was secretly given her powers back by Mr. Mxyzpltk. Mxyzpltk did so to teach Superman a lesson in humility by giving what he thought was an Earth girl, powers mightier than Superman. As a result, Supergirl's powers were returned and she was also made immune to Kryptonite. Furious that Supergirl got her powers back, Lesla-Lar tried to use her device on Supergirl again, but because it used Kryptonite to take away Supergirl's powers in the first place, it had no effect on her. Her use of these devices were finally tracked by the Kandorian police who promptly arrested her for the illegal use of the rays, and destroyed her devices, foiling any plans she may have had in the future. 
Lesla-Lar would return in two other instances. She would briefly possess the body of Lena Thorul, and working with Phantom Zone criminals in collecting illegal Kryptonian weapons. This would prove to be her downfall as the criminal Kru-El would test the devices on her disintegrating her body.Her consciousness would survive and deluding itself into thinking it was Supergirl's sister, tried to take over the Girl of Steel's body. She would temporarily succeed, however the Danvers would detect the differences in their adopted daughters personality. Upon calling Lesla-Lar on her true identity, she weakened Lar's control enough for Supergirl to regain control and expel Lesla-Lar's consciousness from her body, seemingly destroying it.
While on Earth, Lesla-Lar gained the same super-powers any Earth-One Kryptonian gained while on the planet Earth. This included invulnerability, super-human strength, flight, x-ray vision, etc. She shared the same weakness to Kryptonite as well while on Earth. While in the city of Kandor, she did not have these powers as it's gravity approximated that of Krypton. As a disembodied consciousness, she had the ability to invade the minds of others, take control of their bodies and expel their consciousnesses and keep them out.
Lesla-Lar used a number of different scientific devices, primarily a ray which could sap a Kryptonian's super-powers using a ray of Kryptonite, a device that allowed her to switch places with someone on Earth (sending that person to Kandor and adjusting both beings heights accordingly) and a device that would mentally condition people. She'd commonly use the latter to make Supergirl believe that she was really Lesla-Lar. This device had a fail safe switch that Lesla-Lar would wear on her wrist that would reverse the process and her mental conditioning.
This character is or was primarily an enemy of Superman in any of his various incarnations, or members of the Superman Family. This template will categorize articles that include it into the "Superman Villains category."