•
Gabriel Damon (Takuma Gōno in the Japanese adaptation) as
Nemo, a human boy living in
New York City who is taken to Slumberland to be the official playmate of Princess Camille. In actuality, however, he is being summoned to be the heir to the elderly King Morpheus. He is given the key to Slumberland but is warned by the king to leave a door with a coiled dragon emblazoned on it closed. Sadly, he opens the aforementioned door when he is tempted by Flip and goes on a quest to restore Slumberland to its rightful glory, save King Morpheus and defeat the Nightmare King. •
Mickey Rooney (
Chikao Ōtsuka in the Japanese adaptation) as
Flip, a
clown who is described as a "frightful fellow" by Professor Genius, he is wanted throughout Slumberland for "having fun" (the bounty on his head is a sizeable one) and his only friend is his partner-in-crime: a bird named Flap. He tricks Nemo into accidentally releasing the Nightmare King and blames Nemo for the ruin of Slumberland. He is in possession of a map of Nightmare Land (hand-drawn and written in his own special code) and serves as the guide to the Nightmare Castle until he is replaced by the Boomps. He has a serious smoking addiction. In the real world, he is a clown in a circus that stops in Nemo's town. •
René Auberjonois (
Kōichi Kitamura in the Japanese adaptation) as
Professor Genius, King Morpheus' advisor. He comes to the real world to bring Nemo to Slumberland. As a sophisticated man, he is quite punctual and prefers order as opposed to madness. He is quite a dancer, as he dances quite a bit during Nemo's coronation ceremony. In the real world, he is an organ player in a circus that stops in Nemo's town. • Danny Mann as
Icarus, a
flying squirrel, Nemo's best friend and a supporting protagonist. Icarus is Nemo's only friend from the real world. He shows great concern for Nemo's wellbeing in a sense similar to that of two siblings. He speaks a mix of both squirrel and some English. His screech is painful to the ears of the Boomps. He detests being called a "little rat" (which Princess Camille mistakes him for). Unlike other squirrels, Icarus eats human food, like cookies. His initial relationship with Princess Camille, though rough, eventually changes for the better. •
Bernard Erhard (
Kenji Utsumi in the Japanese adaptation) as
King Morpheus, the sovereign of Slumberland. He has protected Slumberland for years with the help of the royal scepter: an ancient weapon of great power. Though he is a child at heart, he knows when to be serious. He has Nemo brought to Slumberland so that he may become his heir to the throne. He gives Nemo the key to Slumberland, which can open any door; however, he warns Nemo of one door with a dragon symbol emblazoned on it that must never be opened. Like Professor Genius, he is quite a dancer, as he dances alongside the Professor during Nemo's coronation ceremony. When Nemo accidentally releases the Nightmare King, King Morpheus is captured and Nemo has to go and rescue him from Nightmare Land. In the real world, he is the
ringmaster of a circus that stops in Nemo's town. •
Bill Martin (
Tarō Ishida in the Japanese adaptation) as
The Nightmare King, a demonic horned creature who rules over the realm of Nightmares or Bad Dreams. When Nemo goes to Nightmare Land to save King Morpheus, the Nightmare King has his minions capture Nemo's friends (Professor Genius, Flip, and Princess Camille). He is shown to be a liar and quite temperamental as he destroys several minions for the failure of just one of his underlings (the general of his army). Among his forces are a gigantic flying stingray and a presence known as "The Nightmare". The only thing that can vanquish him is the royal scepter. • Laura Mooney (
Hiroko Kasahara in the Japanese adaptation) as
Princess Camille, the daughter of King Morpheus. Though she initially acts somewhat spoiled, she eventually grows to like Nemo. She also grows fond of Icarus (and vice versa, despite a rough start). When her father is kidnapped by the Nightmare King, she takes over as sovereign but decides to join Nemo in his quest to save King Morpheus. In the real world, she is riding on an elephant in a circus that stops in Nemo's town. •
Greg Burson (
Tesshō Genda in the Japanese adaptation) as '''Nemo's father''' •
Greg Burson as
Flap, Flip's bird companion. •
Jennifer Darling (Mari Yokō in the Japanese adaptation) as '''Nemo's mother''' •
Neil Ross (
Hiroshi Ōtake in the Japanese adaptation) as
Oompa, a member of the Boomps that befriend Nemo. •
Alan Oppenheimer (
Keiichi Nanba in the Japanese adaptation) as
Oomp, a member of the Boomps that befriend Nemo. •
John Stephenson (
Masaharu Satō in the Japanese adaptation) as
Oompo, a member of the Boomps that befriend Nemo and is the largest of the Boomps. •
Sidney Miller (
Kōzō Shioya in the Japanese adaptation) as
Oompe, a member of the Boomps that befriend Nemo. •
Michael Bell (
Hiroko Emori in the Japanese adaptation) as
Oompy, a member of the Boomps that befriend Nemo. •
Kathleen Freeman (Kimie Nakajima in the Japanese adaptation) as
the dance teacher • Bever-Leigh Banfield (Seiko Nakao in the Japanese adaptation) as
the woman •
John Stephenson (Kazumi Tanaka in the Japanese adaptation) as
the dirigible captain •
Bert Kramer (
Yukimasa Kishino in the Japanese adaptation) as
a goblin, a hideous creature that serves as a member of the Nightmare King's army. They are sent by the Nightmare King to ensure that Nemo doesn't reach his castle and free King Morpheus. Though the goblins succeed in capturing most of Nemo's friends, they fail to capture Nemo himself and, when the Nightmare King finds out, he kills them all in a fit of rage. The only goblins to survive are the Boomps (who, in contrast to the other goblins, are not hideous and are actually good). •
Beau Weaver (Tarō Arakawa in the Japanese adaptation) as a
police officer •
Sherry Lynn as
Bon Bon, a
Page in Slumberland who works for King Morpheus. In the real world, he and his fellow pages are the drivers of the wagon with the organ in a circus that stops in Nemo's town. • Guy Christopher as a
Courtier and a
police officer •
Nancy Cartwright and
Ellen Gerstell as the
pages, servants of King Morpheus. In the real world, they and Bon-Bon are the drivers of the wagon with the organ in a circus that stops in Nemo's town. •
Tress MacNeille as an
elevator creature •
Michael McConnohie as an
etiquette master •
Michael Gough as a
teacher • Michael Sheehan as a
fencing master •
June Foray as the
librarian •
Gregg Berger as the
equestrian master ==Production==