When the player fulfills certain requirements, the Post Officer will give out a Subpoena containing a Bailiff with a certain level. The level of the Bailiff corresponds to the level of the Item World on which can be found a Court Gate. Entering the gate with one or more characters (to get more than one character through the gate, stack up a tower of characters and throw them into the gate) will allow those characters to receive a Felony. The Bailiff will vanish once this Gate is used.
As Bailiffs are pre-subdued, the player can move them from one item to another, but 2 Bailiffs cannot be present on a single item at the same time. Naturally, you'll want to move them to an item worth leveling up (like a Legendary item) and never actually enter the Subpoena worlds.