On the first round of Planet Sonata, after delivering the last metronome to the bird robot, if you sit and wait for nearly a minute before going to the boss, a rendition of the nursery rhyme "Mary Had a Little Lamb" will start playing.

The Japanese version had seven additional passwords that were taken out of the international versions:

• AAAAAA: Gives infinite continues.
• CANDY: Disables collision damage.
• FEEL: Enables invincibility, true sight, and stage select.
• MACCHA: Changes the color of Greedy's Castle from brown to green. The word "matcha" or "maccha" means "powdered green tea" in Japanese.
• MASTER: A simple text saying "PLEASE WAIT FOR NEXT STORY THANK YOU..." appears on the password screen. Nothing else happens.
• STAR: A shooting star flies in from the top-left corner of the screen to the bottom-right corner on the password screen.
• VALDI: Jumps to an interactive version of the Valdi star system seen in the credits.

The boss for the Planet Freon in the Japanese version looks like a cat monster. This was changed to a frost monster in the International version, as the cat monster is a reference to a Japanese Idiom. A person who doesn't like hot or spicy food can be said to have a "cat tongue", which is why you throw hot soup in his mouth to hurt him.

While Ristar looks cheerful in the Japanese version, Ristar looks angry in the international version. This concept is similarly used in the Kirby series' game box art.

The miniboss of Planet Sonata, known as Neuos in Japan, is easier in the Japanese release. Before the bird heads fall down, a musical note will appear on their forehead to show the player which one they're supposed to attack. In the international release, the developers removed the musical notes.

According to Akira Nishino, there were plans to make a sequel to Ristar. They got as far as making character designs, but it was never developed further for various reasons, and was eventually dropped.

The character of Ristar started out as an early concept for Sonic the Hedgehog. The character was going to be a rabbit that could grab the environment and enemies with his ears. As the game play got faster and faster, the character was shelved and replaced with Sonic. The character was later picked up to be in a game called "Feel" before being changed into a shooting star and both the game and the character's name were changed to Ristar.