The main character and protagonist of the story. He suffers from constant nightmares. Nemo is around the age of eight and is a generally sweet boy who loves the circus. However, he has been known to lose his head now and then, such as when the princess makes fun of his pajamas and refers to Icarus as a "little rat." He has been summoned to Slumberland by Professor to be the Princess's playmate and (as he finds out later) the future prince of Slumberland.
A professor and a genius, Professor Genius is something of a right-hand man to King Morpheus. He tries his best to help and watch over Nemo, particularly making sure he avoids Flip. He is extremely well mannered and though he tries to maintain a formal front wherever he goes, the Professor can't resist the urge to dance whenever the chance presents itself.
The king of Slumberland and the Princess's father. He ordered the Professor to bring Nemo to Slumberland to be his daughter's playmate. He shares an interest of trains and machinery with Nemo, who grows very dear to him. The king, in spite of his expected responsibilities, has a habit of sneaking off to his playroom (where he accidentally runs into Nemo who, at the time, wasn't aware of who he was). Morpheus decrees that Nemo is to be trained to be Prince of Slumberland and gives him "The Golden Key" which can gain him access to anywhere in Slumberland (provided Nemo promises not to open the door with the same symbol as the key on it). He gets kidnapped by the Nightmare King at Nemo's coronation, prompting Nemo to set off after him, along with the Princess, Prof. Genius, Icarus, and Flip.
The story's main antagonist. He was sealed behind a large door long ago, until Nemo, under Flip's misguided influence, accidentally releases him. He kidnaps King Morpheus in order to lure Nemo, and more importantly the Royal Scepter (which he and the other inhabitants of Nightmare Land fear), to his Nightmare Castle. Nemo barely manages to destroy the Nightmare King with the scepter and ends the evil king's reign along with Nemo's own nightmares.
One of Slumberland's black sheep characters and an extraordinary troublemaker. He is the first person Nemo meets in Slumberland. Flip enjoys causing mischief and smoking cigars (much to the distress of the others), and travels around Slumberland on a giant crow named Flap. According to Flip, he is the only person in Slumberland wanted "for having fun" (he even has a wanted poster hidden under his coat which reads that he has a $1,000,000,000,000,000 reward for his capture). He is also the owner of the only known map of Nightmareland. Even though Nemo usually finds his antics funny, most of the others are angered by him, the Princess even hitting him on more than one occasion in the film. He's the one who convinces Nemo to open the door with The Nightmare King behind it (even though he meant no harm by it, not knowing what the door held), thus leading to the story's main events.
Usually she is simply called the Princess. Camille is around the age of ten. She was lonely, being an only child and her father always gone on business, thus one of the main reasons Nemo is brought to Slumberland is to be a companion for her. After a few bumpy starts, Nemo and she become friends. She detests Flip and his constant cigar smoking (and decrees that he is never to smoke another in her presence). In spite of his frequent absence, she dearly loves her father and commissions Flip (at Nemo's advice) to take them to Nightmareland, after King Morpheus is kidnapped by the Nightmare King. Flip refuses at first until he discovers that his only alternative is to be shot out of a cannon into space. At the film's end the Princess shares a kiss with Nemo before he wakes up.