Lionel's Earth-2 doppelganger.
After the revelation of being a Luthor, Tess (Lutessa) becomes Lionel's heir and inherits his fortune and treasures as a result which included a kryptonian mirror box which Clark accidentally used to travel into a parallel universe where he finds his counterpart is the son of an alternate-reality Lionel Luthor (the parallel Earth's de-facto supreme ruler, or at the very least the de-facto supreme ruler of the parallel world's Metropolis). Although Clark eventually finds and uses the mirror box again to return to his universe, the Lionel from that reality also escaped.
The alternate-Lionel finds Lionel's journals to learn everything he can about his doppelganger's history.
Lionel was mentioned several times. Lionel's death by Lex was mentioned by Tess in a negative way while the-alternate Lionel looked at it was the best thing Lex ever did. Clark compares Lionel's pre-redemption influence which "destroyed" Lex to the alternate-Lionel's influence to Conner Kent. Later, Tess used her birth certificate with Lionel's fingerprints on it to prove that the alternate-Lionel was a "fake".
After Tess poisoned Lex with a neurotoxin, Lex had completely forgotten his experiences with Lionel: being raised by him, being declared "insane" and put in an asylum by Lionel, feeling betrayed due to Lionel's closeness to Clark, and even murdering his father.