The Completely Sane Man is a minor character in Problem Sleuth who originated in Jailbreak, where he appeared only as a skeleton.
CSM is one of the Prisoners living in the prison compound in Jailbreak. He has an odd variety of possessions, including a violin, a boot and several Keys. His imprisonment is solitary until Zombie Ace Dick crashes in through his ceiling, at which point the two become good friends and work together to escape. Kind of.
In the epilogue, the Completely Sane Man gets married to Wifehearst. His wardrobe includes a rather nice top hat, which he seems to have stolen from his acquaintance. It is not quite certain where the Completely Sane Man marries Wifehearst, but he is wed by the Weasel Emissary.