Vilandra is Isabel Evans's original name, when she was princess of the planet Antar. Vilandra was the sister of Zan, the beleaguered King of Antar, and was widely regarded, even by her enemies, as the most beautiful woman in the Five Worlds.
Princess of Antar
Vilandra was betrothed to Rath (later Michael Guerin), her brother's second-in-command, but she fell in love with Zan's mortal enemy, the political upstart and later rebel leader Kivar. Vilandra's love ultimately led to the downfall of her family, and it was she who opened the gates of the capital to Kivar's army. Vilandra was killed, possibly by accident, in the subsequent battle, but her DNA was salvaged by her mother and used to create a hybrid body for her to live on Earth.
The Royal Four and their "Dupes"
The initial experiment that created the Royal Four's hybrid bodies resulted in clones that were considered "defective" by their shapeshifter guardians, so a second set was created without these flaws. After learning that Kivar had sent agents to Earth to retrieve Vilandra and destroy Zan and Rath, both sets of hybrids were sent to Earth to maximize the chances of the Royal Four's success.
Although "Lannie", Vilandra's New York hybrid, remembered her life as Vilandra and desperately sought to resume it, Isabel, the Roswell hybrid, had no memory of her life and was mortified upon learning about the damage that Vilandra had caused with her selfishness.
Isabel Learns the Truth
In a confrontation with Congresswoman Whitaker, Isabel was told of Vilandra's history and of her true allegiances. Isabel destroyed Whitaker, but was shaken by the revelations, as she couldn't picture herself betraying her friends and family the way Vilandra did. Isabel kept her discovery to herself, but took any opportunity she could to piece together more of Vilandra's history.
The others eventually discovered truth about Vilandra when Lonnie and Rath arrived in Roswell to convince Max to join the summit of alien factions in New York. Lonnie and Isabel swapped notes on Vilandra's history, with Isabel become even more disillusioned with her former self. Lonnie also spilled the beans to Max about Vilandra's history, and an incensed Max agreed to attend the conference while leaving Isabel in Roswell. Max ultimately forgave Isabel for lying to him, and assured her that whatever Vilandra had done, it didn't change the fact that Isabel was his sister. This contrasted with Zan, who always held something of a grudge against Lonnie for Vilandra's actions.
More than a year after the aborted peace summit in New York, the shadows of Vilandra's personality began to affect Isabel. She began to have vivid, erotic dreams involving an attractive blond human male in the weeks leading up to her wedding. To her horror, she learns at her wedding that the man from her dreams is Kivar's Skin, and that he has finally ventured to Earth in person to reclaim her.
Max and Michael covertly join Isabel on her honeymoon to protect her and Jesse from Kivar. They ultimately fail, as Kivar's touch is enough to restore Vilandra's personality. Vilandra fights them off and prepares to enter the wormhole that will return her to Antar, but Jesse's sudden appearance and Michael's equally-sudden memories of what really happened when Vilandra let Kivar into the city caused Isabel's personality to reassert itself. She sent Kivar back to Antar, warning him that she would kill him the next time he returned, and apparently sealed the wormhole.