Ghost (Super Mario Galaxy 2)
A ghost is a helpful character that appears in the Wii game Super Mario Galaxy 2. A ghost can be created at any point in the game, but will only be created when the player successfully completes any mission in a galaxy while playing as Luigi instead of Mario or by having 9999 Coins in the Coin collection in Starship Mario, in which would cause a ghost to appear in all galaxies, all missions leaving no need to play as Luigi. For this reason, it is considerably easier for the player to create ghosts after Mario has defeated Bowser in Bowser's Galaxy Generator at the end of the game, as Luigi now appears onboard Starship Mario, and is available for the player to use at any point. Before this point in the game, ghosts can only be created if the player agrees to let Luigi take over a particular mission when he appears at the start of some galaxies. Therefore, the player has more control over where and when ghosts are created later in the game, as opposed to earlier in the game.
After the player has completed a mission with Luigi, and upon returning to Starship Mario, the player will receive a message saying that a ghost has appeared in the galaxy from which they just came. The ghost that is created will always take the form of a translucent Luigi, and will never take the form of Mario. Once created, a ghost can be found at the starting planet / area of each galaxy for which the player has followed the procedure to create it. To activate the ghost, the player must have either Mario or Luigi touch it as it runs in place near the start of a galaxy, after which it will take off running through the galaxy, leaving a trail of green arrows on the ground behind it. Should the player ever lose sight of the ghost while it runs through the galaxy (as it does move considerably faster than Mario), they can use these arrows to track the ghost's previous movements.
The ghost will usually take any and all available shortcuts while running through a galaxy, even when they are not readily apparent to the player. This is because the ghost reflects the actions of some of the game designers as they had gradually completed the galaxy themselves, much like the actions of a Staff Ghost in Mario Kart Wii. Depending on the mission in a galaxy, the ghost may either lead the player to the Power Star at the end of the mission, or lead the way to a hidden Comet Medal in the galaxy. If the ghost leads the way to a Power Star, it will jump up into the air where the Power Star is and grab onto the bottom of it, turning around with the Power Star as it rotates in midair (if the Power Star has not appeared yet, it will merely hover in the air until it does, at which point it will grab onto the Power Star). If it leads the player to a Comet Medal, it will lie down on the ground under or near the Comet Medal, and will not move for the remainder of the mission. Alternatively, the ghost may lead the player to neither of these locations, in which case it will simply run to a hard-to-reach area, and lie down on the ground and relax.