Cap'n Matey appears to be a fat, ugly precursor of our beloved Bleinmeis: he appears in two different locations, and Bane will allow you to fight (and kill) him twice. Here's how it works:(1) When you first enter the Captain's Den, tell Matey that you want to "fight"; after the fight starts, run away. (I suggest you "hide" all characters, first. Cap'n Matey has a mean attack.)
(2) Go into Matey's chambers -- he's sleeping there. Open the treasure chest and fight the good fight. (You CAN'T run from this one.)
(3) Go back to the main room. When you enter the entrance square, you will again encounter and fight Matey.
Unfortunately, all you get from the extra fight is another load of experience and the Captain's gear.