Lucas Luthor is a supporting antagonist in the second season of Smallville, being alluded to in the seventh episode before appearing as the secondary antagonist of the episode in which he appeared.
He was born from an affair between Lionel Luthor and Rachel Dunleavy and the paternal half-brother of Lex Luthor and Tess Mercer. However, Lionel ordered Rachel to give the baby up for adoption to maintain his reputation and avoid scandal.
Years later, Rachel initially believed Clark to be her long-lost son. However, she learned he wasn't when he survived getting fatally stabbed by an ax.
However, Lucas was found by Lex Luthor, and they confirmed to stage a hostile takeover for LuthorCorp from Lionel.