Yo-kai are spiritual beings whose influences are responsible for almost every one of the strange and paranormal events in the human world. Most of these influences range from sudden changes in the behavior of people or abnormal weather in otherwise normal days, as well as unexpected landscape changes or control over a different force of nature.
With some exceptions, most Yo-kai are invisible to humans without the help of an item such as the Yo-kai Watch. In some cases, Yo-kai can join human societies by putting a magical leaf on their heads, as seen by Komasan, Komajiro and Rawry in the anime; through more conventional forms of shape-shifting, such as Kyubi, Mimikin and Robonyan; possesion of others, such as Infour; or even choosing to manifest themselves at will in a certain way, such as Lord Enma and Toiletta. Also in the anime, some Yo-kai are made permanently visible due to the circumstances of their death or the origins of their Yo-kai conversion, such as Pandanoko, Manjimutt and Snartle.
Most of them being based on the actual youkai and other mythological entities from traditional folklore and even certain fictional characters from popular culture, most Yo-kai have unusual body compositions, such as being single- or multi-eyed or even multi-limbed; others may be based on inanimate objects, which are known under a sub-category of Yo-kai known as Tsukumogami (Japanese: 付喪神) or even food, also ranging from being based on animals like mammals, birds, fish or insects and even more traditional demons or mythical creatures from folklore.
Many Yo-kai used to be humans, such as Frostina/Blizzaria, Hungramps, Spoilerina and Venoct; animals, such as Cricky, USApyon, Tomnyan and Jibanyan; or even inanimate objects, such as Wazzat, Dummkap and Brokenbrella. Though the process for becoming a Yo-kai is largely unknown, most Yo-kai develop an attachment to an important place or part of their previous lives, with some having unsettled business to fulfill; however, doing so does not make them pass to the other side.
Though most Yo-kai, technically being ghosts, cannot die in the strictest sense of the word, they can have their Soul Gems forcefully removed, leaving an empty shell behind. The "death" can be reversed by returning their souls, however. Those who can or are able to die simply have a supernaturally long life span. It was also revealed that some Yo-kai can in fact be exorcised and sent to the other side.
It is not yet clear when the first Yo-kai came into existence, though several of them were around since the Sengoku period and Edo period. A group of earlier Yo-kai, known as the Classic Yo-kai, existed at least 60 years prior the start of the series, along with other Yo-kai which can also be found in the present, with others being encountered only in certain time periods. Likewise, Merican Yo-kai can be found in St. Peanutsburg, U.S.A., implying that other areas of the world have their own individual Yo-kai, not just Japan.
Tribes of Yo-kai
- Main article: Yo-kai Tribes
Originally, every kind of Yo-kai were divided into the eight main tribes: Brave, Mysterious, Tough, Charming, Heartful, Shady, and Slippery. In the video games, each of the tribes has their own strengths and rarities, and if two or more Yo-kai of the same tribe are adjacent to each other, they will receive a Tribe Unity bonus. In Yo-kai Watch 2, the Wicked were introduced, consisting of the Wicked Executives and the Wicked Servants. In Yo-kai Watch 3, the Hagure debuted as the 10th main tribe with Android Yamada being the first from the tribe to officially be revealed so far, and the Yo-kai Watch Busters subseries introduced the Enma. Every tribe has their own summoning unique chant when a Yo-kai from that tribe is summoned.
- Classic Yo-kai (Japanese: 古典妖怪 Koten Yōkai): Yo-kai introduced in Yo-kai Watch 2: Bony Spirits and Fleshy Souls which are wholly based on the youkai of classic Japanese mythology. They can only be found in the past.
- Merican Yo-kai (Japanese: メリケン妖怪 Meriken Yōkai): Yo-kai introduced in Yo-kai Watch 3: Sushi and Tempura. As the name implies, they are from America. They are considered a subgroup of Present-Day Yo-kai.
- Legendary Yo-kai (Japanese: レジェンド妖怪 Rejendo Yōkai, Legend Yo-kai): A group of powerful Yo-kai that can only be befriended after filling out the Legend Pages in the Yo-kai Medallium, which is done by collecting certain sets of Yo-kai Medals to break the seal surrounding a specific Legendary Yo-kai, with each seal requiring a different set of Yo-kai. Like every Yo-kai, a Legendary Yo-kai falls under one of the tribes. Unlike every other kind of Yo-kai that from their respective tribes, Legendary Yo-kai's summoning chant are very unique, bombastic and grandiose.
Merican Legendary Yo-kai
- Merican Legendary Yo-kai (Japanese: メリケンレジェンド妖怪 Meriken Rejendo Yōkai, Merican Legend Yo-kai): A group of Legendary Yo-kai from America, like Merican Yo-kai, introduced in Yo-kai Watch 3.
Great Legendary Yo-kai
- Great Legendary Yo-kai (Japanese: 偉人レジェンド妖怪 Ijin Rejendo Yōkai, Great Legend Yo-kai): A special group of Legendary Yo-kai in Yo-kai Watch 3, who were originally famous people before their death. Notably, all of them sport a five-pointed sheriff-like star with the kana イ, meaning "I", which stands for "Ijin" (偉人, Kanji for "Great Man").
The Yo-kai Watch
Models of the Yo-kai Watch
- Main article: Yo-kai Watch (item)
The Yo-kai Watch (Japanese: 妖怪ウォッチ Yōkai Wotchi) is a device that allows the humans to see and communicate with Yo-kai; the Yo-kai can also communicate with humans.
Yo-kai Watch Model Zero
The Yo-kai Watch Model Zero (Japanese: 妖怪ウォッチタイプ零（ゼロ）式 Yōkai Wotchi Taipu Zero-shiki lit. "Youkai Watch Type Model Zero") debuted in Yo-kai Watch 2 and is a model that was invented 60 years ago, marking it as the first usage of the Yo-kai Watch. Both Z and Classic Medals are used for this model.
Yo-kai Watch U
Yo-kai Watch Dream
- Main article: Yo-kai Medals
The Yo-kai Medals (Japanese: 妖怪メダル Yōkai Medaru) are small coin-like medals used to summon Yo-kai through the Yo-kai Watch; Yo-kai normally give the medals as a sign of friendship. Some Yo-kai who have the original medals are usually silver, and used for the original model of the watch. A Legendary Yo-kai's Yo-kai Medal is identified dominantly by the color gold.