Lucius's Japanese voice actress remains officially unknown. While Japanese voice actors are listed in SoulCalibur VI's credits, their specific roles aren't. However, I can make an educated guess. While Reiko Takagi voices Cassandra in SoulCalibur, she's also had a number of anime roles, including Tadase Hotori—a young boy—in Shugo Chara:
Tadase Hotori (Shugo Chara) vs. Lucius Alexandra (SoulCalibur VI) - Japanese Voice Comparison
Call me crazy, but those two sound too similar to be a coincidence, especially when we account for natural voice changes that come with age. The first season of Shugo Chara started airing in December of 2005, while Cassandra's Soul Chronicle was included as part of her DLC in SoulCalibur VI in 2019. Even Cassandra herself sounds somewhat different between SoulCalibur III and SoulCalibur VI. Come to think of it, Lucius and Tadase even look similar. If true, those conversations between Cassandra and Lucius are actually Reiko Takagi talking to herself...