Baku has the appearance of a purple and pink tapir. He has yellow eyes with red irises and black pupils. He has small, pointy, horns that are a different color purple than his body (but the same color purple as the tuff of fur on his tail). He has an orange flower-like design next to his eyes. He also has sharp teeth on both his upper and lower jaws.
He tends to refer to himself in third person.
Baku has the ability to put his targets to sleep, allowing him to eat their dreams. The dream takes the form of translucent clouds that can also be viewed like a movie if he doesn't immediately eat them.
In the first game, Baku also demonstrates the ability to assume the appearance of a human. This allows the player to go out at night without their parents knowing, as Baku takes the player's place in bed, asleep. Baku's dream eating also removes all traces of tiredness from the player, allowing them to be indefinitely energetic and not feel exhausted.
Yo-kai Watch animation series
In Yo-kai Baku, Baku puts everyone but Nate in school to sleep in order to feast on their dreams. Despite Nate catching him in the act, he refuses to stop, saying that he will only stop after feasting on an incredible dream as he considers all the dreams he's eaten so far as unfulfilling. Eventually Nate summons Shogunyan, whose dream of being eighty feet tall leaves Baku awestruck. After eating Shogunyan's dream, he happily gives Nate his Medal.
Baku is later summoned in Yo-kai Insomni to deal with Insomni, after the Eerie Yo-kai has Inspirited everyone into not getting any sleep the previous night. After their attacks collide, Baku and Insomni appear to have a faceoff in a wrestling arena where the two sleep-themed Yo-Kai duke it out; Insomni eventually triumphs. However, the sequence turns out to be Insomni's dream, and she is revealed to have fallen asleep the instant she was hit with Baku's hypnotic gas.
Baku is automatically befriended after defeating him in the main story. Baku also appears on the Bamboo Path in Blossom Heights.
Yo-kai Watch 2
Baku is automatically befriended again during the main story, this time appearing in the classroom when the player forgot their summer homework in the school. After hitting Whisper with his sleeping gas (thanks to Jibanyan using him as a meat shield) and a resulting fight, Baku offers his Medal in apology.
Baku can also be obtained from Pink Coins in the Crank-a-kai in the past.
Baku is based on the dream eating youkai of the same name. Originally from China, this beast is described as having "a bear's body, small eyes like a rhino, a long snout, strong legs like a tiger's and a spotted pelt": overall it resembles a tapir, and in fact the word "baku" is also used to refer to the animal.
This creature will feed on good and bad dreams alike while people are asleep. Those who are woken up by nightmares will invoke its name to let it devour them so they can peacefully go back to sleep, and it's said that even the general Hideyoshi Toyotomi kept a picture of a baku by his bedside to keep nightmares away. It is also said that anyone who sleeps on a baku's pelt will never get sick.
Baku is male, but in the English version of the anime is referred to as female. This is probably due to his purple coloring and long eye lashes.
Baku's "random" nicknames in Yo-kai Watch 2 are: Dreater, Barnie, Chomp, and Crunchy.