Lucas Luthor is a minor antagonist in Smallville, appearing in the Season 2 episode "Prodigal". He is the son of Rachel Dunleavy and the illegitimate son of Lionel Luthor, and therefore the half-brother of Lex Luthor, Tess Mercer and Julian Luthor.
Lucas has brown hair with dark blonde highlights which are spiked up, gray eyes, slightly tanned skin, and is clean-shaven most of the time.
Lucas is a volatile sociopathic person who is willing to do just whatever it takes to become wealthy and save himself with no remorse. He intentionally tried to elbow Clark just so he could win a basketball game, attempted to fire Lana Lang from the Talon to get back at Clark for gathering information on him, and knocked Jonathan Kent out when he came to kidnap Lex.
Lucas was also willing to betray his family for money; despite this, Lex decided to take care of Lucas since he wanted family intimacy and protected him in the end from their father.
Years ago, Lionel had an affair with Rachel Dunleavy, his wife's nurse, and Rachel became pregnant with his child, whom he then forced Rachel to give up for adoption due to her unstable mental condition. He also paid Rachel a large amount of money to get her to leave him and his family alone. About twelve years later, Lionel located Lucas and anonymously gave him financial support, though he falsely claimed to Lex that Lucas died before his first birthday. Lucas grew up in poverty while being bounced from foster home to foster home.
In the Season 2 episode, Lex finds Lucas in Edge City after a seven-month search and saves his half-brother from some gangsters who are pursuing Lucas because he cheated them at a poker game. Lex takes Lucas to LuthorCorp to meet their father as well as organize a takeover of the company, which Lex hopes to carry out since Lucas, being legal age, is due to claim his inheritance. Lucas, however, betrays Lex when Lionel gives him ten million dollars to sign his shares away, and together, they take Lex's share of the inheritance and throw him out of the Luthor Mansion, forcing him to move in with the Kents.
Lucas realizes that, at his age, he can have a more sizable inheritance than what Lionel is offering. He also harbors anger towards his father for abandoning him. He even realizes that Lionel was never blind and forces him to admit he has since regained his sight by throwing a pool ball at him. Lucas also takes control of all of Lex's assets, including the Talon, and tries to fire Lana Lang just to get back at Clark Kent for collecting information about him.
Clark, while trying to convince Lucas to change his mind about Lana, ends up saving Lucas when one of the gangsters locates Lucas and tries to kill him. Lex discovers that Lionel tipped the gangsters off on Lucas' whereabouts and reveals that to his half-brother. Later, Lucas kidnaps Lex from the Kent Farm, knocking out Jonathan Kent as he does so, and calls Lionel, demanding that he come to his office at LuthorCorp.
When Lionel arrives at his office, Lucas swears to kill Lionel unless Lionel kills Lex, who is taped to a chair. Clark (who, having followed Lionel, hides in an air vent) uses his heat vision on Lucas' gun, causing him to drop it. Lionel then shoots Lucas multiple times in a fit of anger, only to find that his gun is filled with blanks. Lucas and Lex then reveal to their dumbfounded father that they schemed together to prove Lionel is no longer blind and have Lex run LuthorCorp again.
By the episode's end, Lex informs Lionel that he sent Lucas to a place where he would be protected, and Lionel told his oldest son that Lucas was a sociopath and would never change. He was never mentioned or seen in the series again.