Greer Grant Nelson was the recipient of two separate sets of superhuman abilities, one scientific and one mystical in nature. As a laboratory assistant to Dr. Joanne Marie Tumolo, Nelson became involved in a physiological conditioning program that would enable a human being to attain his or her full physical and mental potential. Out of financial desperation, Dr. Tumolo had accepted private funding for the project from Malcolm Donalbain, an eccentric who planned to use her conditioning equipment in his nationwide chain of health clubs. Not trusting the test subject that Donalbain had coerced her to use, Tumolo had Nelson secretly undertake the experiment alongside the other woman. Donalbain's test subject, Shirlee Bryant, and Greer Nelson emerged from the battery of treatments with superhuman physical capabilities. Dressed in one of Donalbain's specially designed cat costumes, Bryant died in a test of her abilities. Donalbain had his henchmen set off dynamite charges in the laboratory to make it appear that Bryant had died accidentally in an explosion. Tumolo was injured in the explosion. Believing her mentor to be dead, Nelson determined to stop Donalbain's plans. She donned a stolen costume, and calling herself the Cat, thwarted Donalbain's plans and forced him to commit suicide. Nelson soon learned that Tumolo had not been killed but had been hospitalized for apparent brain damage. During this time, Nelson, as the Cat, pursued a brief career as a crimefighter, operating mostly within the Chicago area. Unknown to Nelson, Tumolo was a member of the Cat People, a race of humanoids magically evolved from cats in Europe during the Middle Ages. Prior to the explosion Donalbain had engineered, Tumolo had become aware of a plot by the subversive organization HYDRA to steal a bacterial culture that had been developed by Tumolo's ancestors. HYDRA had discovered the secret of Tumolo's ancestry and suspected her of possessing the formula for 'The Final Secret.' Tumolo had chosen to feign brain damage in order to evade HYDRA, but HYDRA abducted her from the private home where she had apparently been convalescing. Nelson put on her Cat costume and pursued them. HYDRA agents fled from the Cat's attack, but during the confrontation managed to shoot her with 'alpha radiation.' To save Nelson's life, Tumolo and the Cat People mystically transformed her into the legendary half-human, half-cat warrior they called the Tigra. As Tigra, Nelson helped the Cat People defeat the HYDRA agents. Although she was able to use the cat's-head amulet to change back to her human form, Nelson became so accustomed to and enamored of her feline form that she seldom made the transformation. Moving from Chicago, she became a full-time adventurer, encountering and defeating such menaces as the Rat Pack, Kraven the Hunter, Tabur, and the Super-Skrull. She also briefly worked with Red Wolf, Spider-Man, and the Fantastic Four. Tigra was among the handful of costumed crimefighters that the telepath Moondragon coerced into vying for Avengers membership. She was elected to membership and served for several months, all that time doubting she was worthy of membership in the group. Although she proved herself while in battle with the Molecule Man, she decided to resign her active membership. Moving to San Francisco, she became friends with Jessica Drew, the original Spider-Woman. Months later, when Hawkeye moved to Los Angeles to form a West Coast branch of the Avengers, she was invited to join. She made a staunch addition to the team, despite the fact that the human and feline sides of her personality had become at odds with one another, causing her behavior to become somewhat erratic. Because of her catlike need for attention, she became involved with teammates Henry Pym and Wonder Man at the same time. The ultimate solution to her discordant nature presented itself when she and the Avengers journeyed to the 'Land Within,' the magical dimension where the parent tribe of the Cat People dwelled. The ruler of the Cat People offered to magically cure her of her split personality if she would kill Master Pandemonium, a demonic human being whom the Cat People feared and hated. Although Tigra agreed, she could not bring herself to violate the Avengers' code against killing when she had the opportunity to do so. After almost killing Hawkeye in a training exercise, Greer decided to abandon her Tigra form permanently. Almost immediately, Hank Pym and Greer were kidnapped on behalf of the demon Allatou, who wrongly believed them to be agents of her enemy, Master Pandemonium. The Avengers sought aid from Hellstorm and Hellcat, who were retired heroes at the time, having set up an occult investigations business. Together, the heroes invaded Allatou's realm with Pandemonium on their heels. A three-way fight broke out until Allatou managed to strand everyone with Pandemonium aboard a boat running through the various realms of hell. Shortly after, the castaways found that they had drifted into the Cat People's Land Within. It turned out the Cat People were demons, having become such through their longtime residence in the demonic realms. The Cat People were at the time guarding a shard of Pandemonium's soul on behalf of the demon Mephisto, and they had ordered Tigra to kill Pandemonium to protect it. Tigra continued to refuse to slay Pandemonium, and the Cat People punished her by stripping away her cat-soul altogether and leaving her human. Hellcat, however, owning a version of Greer's original Cat costume, gave it to Greer, who led the Avengers in battling the Cat-People and single-handedly defeated their champion, the Balkatar. The cat-soul was released from confinement in the struggle, and stronger than ever, re-inhabited Greer again, producing a Tigra that was more physically formidable than ever but controlled by Greer's human intellect. The Cat People surrendered, unwilling to battle a 'whole' Tigra who now exactly resembled the revered Tigra of their legends. Returning with the Avengers, Tigra continued as an active member and forsook her previous romantic involvement with others of her team. The Avengers were rocked by controversy when their member Mockingbird revealed that she let the villainous Phantom Rider die in a previous encounter. Sympathetic to Mockingbird's cause, Tigra joined Moon Knight in briefly splitting from the team to accompany Mockingbird, who quit the team and separated from her husband and leader, Hawkeye. Together, the threesome joined Bill Foster (previously Giant-Man) to stop the villainous High Evolutionary, and later fought the Night Shift. Afterwards, the heroes sought help from Hellstorm and Hellcat to rid Mockingbird of the ghost of the Phantom Rider. The three parted ways after this, and Tigra returned to the Avengers. Over time, however, Tigra's cat-soul continued to reassert itself. As it gained more and more prominence, Tigra once again became moody and aggressive, but also began to change physically to resemble that of a predatory cat, and was even discovered to be chasing mice. On a subsequent adventure, Pym was forced to used his size-altering powers to shrink Tigra to the size of a house cat, which she resembled. He imprisoned her until he could discover a cure. Shortly afterward, the time-traveller Immortus freed Tigra to further distract the Avengers from his plans against them, and Tigra roamed the city for some time. The sorceress Agatha Harkness, who at the time was living at the Avengers' compound, discovered and re-captured her. Seeking a cure for Tigra's condition, Harkness called upon the Cat People. The Cat People's new ruler -- Tigra's old foe Tabur, a cat artificially evolved into humanoid feline form -- appeared and restored Tigra to humanoid form. Tabur tried to coerce her into mating with him to legitimize his usurpation of the Cat People's throne, but Tigra resisted and Harkness mystically transplanted Tigra's cat-soul into Tabur, causing him to revert to the form of a mere cat while allowing Tigra to keep her superhuman form but retain her human personality. Using Tabur's discarded amulet, Greer was even able to switch from Tigra form to ordinary human form and back again whenever she wished. Thus empowered again, Tigra rejoined the Avengers' West Coast branch. Later, on a mission against their enemies, the Pacific Overlords, Tigra was forced to crash-land the Avengers' Quinjet in an Australian wilderness, but was rescued by Native Australians. She elected to stay with her rescuers, taking a leave of absence from the team. She did, however, assist a team of reserve Avengers when the main team was off-planet, fighting amid the Kree-Shi'ar War. Afterwards, she apparently remained in the United States, and was on hand to attend the funeral of Mockingbird. Electing not to stay with the newly re-formed Avengers, Tigra left with Starfox for outer space, searching for hedonistic off-planet adventures. Tigra grew quickly disenfranchised with such a lifestyle, and was relieved when former teammate Moondragon assembled Tigra, Starfox, and other Avengers to battle the alien Infinities. When the crisis was averted, Tigra returned to Earth. Having attempted to settle into a new life over the past few years with the Avengers, Tigra wasn't happy with just being 'part of the team'. She realized she had to settle up with the past. This meant getting to bottom of The Brotherhood of the Blue Fist: an elusive group who might have murdered her late husband. Working undercover as Greer Sorenson, she signed up for training at the New York City police Academy. She rooted out the conspirators and the organization was dismantled. Then, she finished her training and undertook a new career in the police force.