It is not known to evolve into or from any other Pokémon.
Klefki is a small, grayish white Pokémon that resembles a key ring with four keys. It has a spherical head with a small, pink oval on its forehead. Two black circles and a keyhole form its eyes and mouth, respectively. Below its head is its pink, teardrop-shaped body with a white, teardrop-shaped marking in the middle. Extending from the top of its head is a horn that resembles an old-fashioned key with two wide, blunt teeth. Extending from either side of the key is a thin loop that surrounds Klefki. Hanging from the loop are various keys that this Pokémon has collected. Klefki is very tiny, tying with Alolan Diglett and Meltan as the shortestSteel-type.
In the past, Klefki inhabited mines. When the minerals it fed on became scarce, it moved to human settlements. It now feeds on metal ions by inserting its key-like horn into chinks in metal. It likes to collect keys, particularly master keys, and will sneak into people's homes to steal them. While the exact reason for this collection is unknown, it does rattle its collection to threaten predators. It will never release a key it likes, so people will give it important keys as a way to prevent theft. Klefki is the only Pokémon capable of learning Fairy Lock, and in the past claimed Crafty Shield as a signature move as well.
A wild Klefki debuted in Gathering Klefki, which Grace had befriended. It helped her break free from her cells numerous times, and later helped free the Vaniville Town residents by stealing the keys of a sleeping Team Flare Grunt and using them to open their cells. Klefki later acquainted itself with X and his friends, giving X Clemont's glasses and allowing him to deduce the location of Team Flare's base.
Klefki is based on a keychain, and possibly also the pagan tradition of blaming fairies for the loss of small objects such as shoes and keys. It may also be based on the superstition of carrying an iron charm to ward off fae mischief.
Klefki is a combination of clef (French for key) or a corruption of kleptomaniac (in reference to its habit of collecting keys) and key.