Lucien, also known as Lucifer, was a Megan traveler who visited Earth "millennia ago" in search of companionship.
Following the Salem witch trials of 1691, the Megans – including Lucien – were forced to leave Earth and return to their homeworld, Megas-Tu.
In 2269, the USS Enterprise discovered their homeworld, and Lucien was the first to greet them. He did so, however, under the condition (and the ruse) that the other Megans would not learn of the Humans' presence at Megas-Tu.
Following the discovery of the Human presence, Lucien and the Enterprise crew were placed on a Salem-type trial to answer for the crimes of their forefathers. Lucien was also placed on trial for welcoming the Humans to their planet, with the threat of spending eternity in limbo.
Kirk was willing to sacrifice himself to save Lucien, despite his previous aliases and reputation from his time on Earth. In doing so, Kirk proved to the Megans that Humans had changed over the past six centuries. The Megans informed them that they had passed the Megans' test and welcomed them to their planet with open arms.