The complete history and origin of Leopold Stryke has yet to be revealed, including the circumstances in which he created the original Eel's costume. He spent his life as a curator of an aquarium and supplemented his income with criminal activities as the Eel, until being confronted at times by the Human Torch.
Later, he was recruited by Count Nefaria to join his ranks of superhuman lackeys, only to fight the original X-Men. Later still, he acted in a partnership with other criminals, including fellow Nefaria-flunkies Plantman, Scarecrow, and Porcupine. He joined his brother, the original Viper, working for the mysterious Cowled Commander, and afterward, the two brothers would join the first Serpent Squad (despite the fact an eel isn't a snake.) The Eel was on hand for the second Serpent Squad, too, but left soon after finding his brother was murdered by the second Viper. The Eel and the Porcupine became followers of the alien Nebulon, which led to their clash with the Defenders.
Finding a job at the Delazny Studios in Hollywood as a marine specialist, the original Eel continued to moonlight as a super criminal, stealing a disintegration device. Although the criminal Enforcer would steal the device, an operative of the Enforcer, the Gladiator, murdered the Eel.