Leslie Gesneria was a volunteer/mercenary for the Life Foundation, an organization that was preparing for the M.A.D. (Mutually Assured Destruction) fallout of the Cold War and sought to provide a comfortable life for their wealthy clients after the impending nuclear holocaust. Leslie was one of the organization's security force members who was chosen to be made into one of the superhuman police force members for the utopian future that the Life Foundation sought to bring about. This was done by forcibly extracting multiple offspring from the alien creature known as Venom, after which the offspring were artificially aged to maturity. Leslie was bonded with one of these symbiote offspring and became one of the Life Foundation's Guardians.
Leslie Gesneria was created by David Michelinie, Ron Lim, Sam DeLarosa and Al Milgrom in 1993 and first appeared in Venom: Lethal Protector #4.
Her symbiote's name "Agony" came about rather unofficially due to a female symbiote character in a Venom toy line. While fan-based, this name has since stuck as in 2012's Carnage U.S.A. Leslie's symbiote was officially identified by the name of Agony for the first time.
Major Story Arcs
During Venom's time as Lethal Protector of San Francisco, Leslie and her "siblings" were defeated by Venom and Spider-Man. Leslie's symbiote was artificially aged to dust and her body was caught in a massive explosion.
It was later revealed that the symbiotes had survived due to efforts by the Life Foundation. They broke Eddie Brock out of prison and kidnapped him in a last ditch effort to communicate with their alien "others." Unfortunately Eddie refused to aid them, due to Carnage's example that his "children" became psychopathic killers. Shortly after Brock's refusal, Leslie was stabbed from behind and killed. The rest of her symbiote siblings mourned her passing, especially Carl Mach, who sought to kill Brock out of revenge when it was assumed that he was the one who'd killed Leslie. It's later revealed however that Scream, having snapped from her schizophrenia and her symbiote's influence, killed Leslie, believing that the symbiotes and any who host them were evil. She went on to kill Carl and the rest of the symbiote hosts even after her ruse was revealed.
Leslie's symbiote survived, and was imprisoned/experimented upon in the Vault along with its siblings (except Carnage and Scream). The symbiotes eventually merged together, and bonded with a man known as Scott Washington to form Hybrid.
In Carnage: USA, the Hybrid symbiote was revealed to have been forcibly separated behind the scenes. The Agony symbiote, along with its siblings, were rendered catatonic as a result of the trauma of the separation. Covert Government Mercury Team, trained with them to teach them specialized tasks. Agony was teamed up with Lieutenant James Murphy, Navy Seal.
Deadpool vs. Carnage
In Deadpool vs. Carnage, Deadpool stole the symbiotes from a government facility to help him combat Carnage. After defeating Carnage he puts the symbiotes on a dog and instructs it to take them back to the government.
Agony first appeared in video-games as a boss in Venom & Spider-Man: Separation Anxiety, she was fought at the end of multiple levels of that game with various combinations of her "brothers and sister".
In 2013, Agony received an unexpected nod when she, not her Mercury Squad counter-part, was hinted as being in Marvel Avengers Alliance Season 2, VIA the Days of Future Past homage teaser. She, along with Reptil, Razorfist, Captain Ultra, Doctor Bong, Yoshida, Psyklop, Razorback, and others were all tagged with the ominous "Slain" label, much as many of the X-Men were on the cover this image references.