As you eviscerate each exhausting stage's scumbag menagerie and complete its simple tasks (find the key for this door, the missing lever for that mechanism), you'll be able to switch between the two active characters on the fly. The computer takes the reins of the fighter you're not controlling, but lets you issue simple commands to influence their behavior. In practice, your partners seem adept only at getting themselves killed, and aren't too reliable at following directions, so expect them to spend a lot of time standing in a corner and blocking attacks. Irritating as they are, your allies are key to solving the game's forgettable puzzles, and you'll need to draw on each one's strengths - ninja girl Jubei's diminutive, crawlspace-firendly size, for example, or staff-wielding monk Tenkai's creepy ability to speak with the dead - to work them out.
Above: Ohatsu collects some souls that she's "liberated" with her assault rifle