Mirror

A Mirror is a reflective surface that displays the reverse image of what is in front of it. As such, mirrors are typically used for personal grooming. However, mirrors have appeared as specialized objects in two games.

Donkey Kong Country 3: Dixie Kong's Double Trouble!
The first unique Mirror is an item that appears in Donkey Kong Country 3: Dixie Kong's Double Trouble!. It is needed not only to complete a trade, but for solving another puzzle as well.

According to Bazaar, who sells the Mirror at his general store for fifty Bear Coins, the Mirror once belonged to Cranky Kong's great grandfather, indicating that it's an antique of sorts. If Dixie Kong or Kiddy Kong buy the Mirror, they can trade it to Barter for his No. 6 Wrench, which they can then give to Björn so that he can fix his chairlifts.

Baffle is also in need of the Mirror, for he needs it to decode a message. If the Kongs give it to him, he will tell them how to open up a Banana Bird Cave in KAOS Kore. However, this is technically optional, as players who are aware of how to open the cave can do so without speaking to Baffle.

Should a player wish to purchase back the Mirror from either Barter or Baffle, they can do so, provided they pay either one of them ten Bear Coins.

Additionally, the Mirror has some strange effects on Barter and Baffle while it's in either of their possessions. If Barter has the Mirror, it will invert the layout of his swap shop, while if Baffle has it, he will speak backwards.

Luigi's Mansion
Mirrors also appear in Luigi's Mansion, where they serve a very unique purpose. If Luigi examines a mirror with his Game Boy Horror, he will be warped to the Foyer, the main entrance of Luigi's Mansion. Every mirror in the mansion, except the room-length mirror in the Mirror Room, functions the same way. Additionally, if Luigi examines the Foyer mirror, he will simply warp to the same place.

These mirrors can be used as shortcuts or escape routes when necessary instead of walking through the entire mansion. The mirror is the only way to exit the Sealed Room, as the door is blocked and Luigi cannot climb up the chimney, which is the entrance to the room.

Trivia

 * Unlike other games, where hints about certain features are provided by characters or signs, in Luigi's Mansion, the player is never informed that the mirrors can be used to warp to the Foyer. The only way for a player to figure this out is to have Luigi experiment with the Game Boy Horror and accidentally examine a mirror.