1. The Ice King: One of the main antagonists in the original series, the Ice King will return in Fionna and Cake. Despite his often antagonistic role, he has also shown moments of vulnerability and compassion.
2. Marceline the Vampire Queen: A centuries-old vampire and a close friend of Finn and Jake, Marceline will also appear in Fionna and Cake. She possesses musical abilities and has a complex relationship with both Fionna and the Ice King.
3. Lumpy Space Princess: Known for her eccentric and often nonsensical behavior, Lumpy Space Princess will make an appearance in the spin-off series. She is the ruler of Lumpy Space and has a knack for getting into unusual situations.
4. Peppermint Butler: The loyal butler of the Candy Kingdom, Peppermint Butler, will also return. He possesses mysterious powers and knowledge and often serves as an advisor and helper to Finn and Jake.
5. Magic Man: A powerful and enigmatic character, Magic Man is known for his trickery and unpredictable nature. He will make an appearance in Fionna and Cake, adding an element of chaos and unpredictability to the story.
6. Cake's Parents: Fionna's best friend, Cake, has mysterious origins. In the spin-off series, viewers may get a chance to learn more about Cake's parentage and meet her parents.
7. The Banana Guards: The loyal protectors of the Banana Kingdom, the Banana Guards, will also return in Fionna and Cake. They are known for their unwavering dedication to protecting their kingdom and its ruler.
8. Susan Strong: A muscular and adventurous character, Susan Strong is likely to make an appearance in the spin-off series. She is a skilled warrior and a valuable ally to Fionna and Cake.
9. Flame Princess: The fiery and passionate ruler of the Fire Kingdom, Flame Princess, may also make a cameo in Fionna and Cake. Her relationship with Finn in the original series adds an interesting dynamic to the story.
10. The Cosmic Owl: A wise and ancient being, the Cosmic Owl serves as a guide and mentor to Finn and Jake. Viewers may get a chance to encounter the Cosmic Owl in Fionna and Cake as well.
These are just some of the minor characters who are expected to return in the Adventure Time: Fionna and Cake series, expanding the universe and bringing back beloved characters for fans to enjoy.