Spoiler!
Since the FL is able to talk to ghost, the FL met the ML's dead brother. His name is Sierre. Sometime after, they learned the truth about why they were locked up in the basement. The ML's father kept beating him everyday and Sierre is actually sick and only has a few months to live. Sierre, then decided to go to the basement with his brother to help him in the succession of the duchy through his death.