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 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, the green-clad plumber will be instantly 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, is magically connected to the mirror in the Foyer. Additionally, if Luigi examines the Foyer mirror, he will simply warp to the Foyer (i.e. he will remain in the same place). The aforementioned mirror in the Mirror Room is the only way to see the otherwise invisible ghosts in the room.

These magical mirrors can be used to create short-cuts when necessary instead of walking through the entire mansion, return to the main entrance if lost in the mansion, or even, if needed, escape powerful enemies if Luigi can examine the mirror fast enough before being attacked. The located mirror is the only way to exit the Sealed Room, as the door is sealed up and Luigi cannot climb up the chimney, the entrance to the room. Some mirrors are covered with a piece of cloth, meaning Luigi has to remove the cloth with the Poltergust 3000 before he can use the mirror.

Finally, one mirror in the mansion, found in the Storage Room, serves its regular purpose by showing the reflection of a switch on the normally cutaway southern wall.

Wario World
In Wario World, a level named Mirror Mansion is full of mirrors, forcing the player to have to "act opposite" as only Wario's reflection is seen. This also blocks enemies, etc.

Super Mario 64/Super Mario 64 DS
In Super Mario 64, there is a room in Princess Peach's Castle called the Mirror Room.

In Super Mario 64 DS, Mario entered a room containing a large mirror in which he fought King Boo. To defeat him, Mario had to follow the Big Boo's movements in the arena's mirrors then attack him when he came too close. After he is defeated, Mario earns the key to Luigi's room. The Mirror Room from the original Super Mario 64 returns - only Luigi can go "into" it using the Power Flower's abilities to find a secret Power Star and a painting that leads to Chief Chilly.

Yoshi's Island DS
In Yoshi's Island DS, when Yoshi fights in the battle against Hector the Reflector, the Nintendo DS's touch screen acts like a mirror, which means Yoshi can see Hector, as Hector was invisible otherwise. Also, each time Hector got hurt, Hector will send chairs and potted flowers to destroy parts of the mirror, effectivily reducing how Yoshi could see Hector. Also, this is the only time the player can see Invisighouls.

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.
 * Originally, a line of coins led to the wall switch in the Storage Room in Luigi's Mansion. Later in the game's development, programmers decided to remove the trail of coins in favor of the reflective mirror.