Greer Nelson was a S.H.I.E.L.D. scientist working on a sort of.... regeneration serum. With DNA from various animals and the blood of a rare, endangered species of Tiger known as "The Tigra Tiger", she seemingly perfected it. She tested it out on herself before anybody else, and her rare blood type, for some reason, reacted in an odd way. She started growing yellow fur. She grew a tail. She had enhanced strength and agility. Strangest of all, she developed claws and a healing factor. She ran off into the jungle to be with "her fellow tigers". S.H.I.E.L.D. found her in a cave with several Tigra Tigers. She reffered to herself as "Tigra". They convinced her to come back, but she was still... odd. With her powers, S.H.I.E.L.D. decided to ask her to use her powers to face the threats normal S.H.I.E.L.D. agents couldn't face. She accepted, and was put on a team with other superhuman agents of S.H.I.E.L.D., who were later based in the Great Lakes where they became known as the "Great Lakes Ultimates".