Karl Weschler is Castle Finster's Bürgermeister (essentially mayor).
When Karl was a child, he went on a train ride with his father and met his own doppelganger. His father was horrified when he discovered the two playing together because meeting a doppelganger is considered extremely bad luck. Karl lived the next several years of his life in fear that his bad luck would harm those near him.
Karl meets Nancy in 2011 when she is investigating the castle's monster problem. He avoids talking about "the accident", which Nancy figures out was an event a few years ago during a tour of one of the dungeons. The room collapsed and people came close to dying. Karl shuts himself away in the castle, believing his bad luck is coming back to haunt him.
Karl also has a passion for German board games and keeps trying to design them. He pays Nancy to test out his new game, Raid!, and even lets her help him create a card for it.
When Nancy catches the monster, Karl feels as though his luck has changed and moves on from the castle and in his life, and maybe goes to Alberta to visit his girlfriend, Mildred. Raid! takes off and becomes successful.
Raid! seems to have become very successful by 2014, as an electronic version of it is available in the Puzzle Palace on the set of Pacific Run: New Zealand.