Keywords are a vital part of the crew hiring process. Depending on which Master or Henchman you select as your Leader, you can hire any characters that share any Keywords with them regardless of their faction for no extra cost. Anything hired outside of Keyword that does not have the Versatile characteristic costs +1 extra soulstone.

Masters are the most powerful characters in the game with many strong and unique actions and abilities. They're most often seen leading a crew, but can be hired alongside other characters to make your crew even more daunting.

"Upgrade Cards represent special options for your Crew. This could be specialized spells prepared just for the battle, unique or rare equipment, or special tactics."
-Malifaux Rules Manual

Many characters have alternate and special edition models that can only be obtained outside of their normal boxes.