Thief
In Chip's Challenge 1, a thief, often referred to as a spy, removes all boots that Chip has when he walks on him, and acts as a wall towards monsters and blocks.
In Chip's Challenge 2, the item thief or red thief (in the base tile set) removes all boots and items that Chip or Melinda have when they walk on him, and the key thief aka blue thief removes all keys that Chip or Melinda have when they walk on him. Instead of stealing their inventory, they can be satisfied with a bribe from the player inventory, which will be removed instead, and leave the other items (including other bribes) or keys untouched. Both these thieves halve Chip or Melinda's bonus points upon being stepped on in cases where they steal items or keys from them. They act as walls for dirt blocks but ice blocks and directional blocks can be pushed over them.
Sometimes, it can be to Chip's advantage to lose a superfluous boot; see Elementary.
Trivia
Microsoft Visual C++ v1.52 contains tools named Spy and DDE Spy. The icon of Spy is identical to the tile used in MSCC; DDE Spy uses the same icon, modified with a lightning bolt.
Spy About dialog
DDE Spy icon
In later versions of Visual C++ / Visual Studio, Spy is replaced by Spy++, with a modified icon:
Spy++ About dialog