Princess Bubblegum could also be the main character. She's smart and scientific. If she were the main character in a love potion fanfic, she could be creating the potion for some noble reason like trying to bring peace to warring factions in the Land of Ooo by making them love each other, but then things spiral out of control.
Finn would be a great main character. He's always getting into wacky situations and his naivete could lead to some really interesting scenarios with a love potion. For example, he might mistake the effects of the potion for true love and go on a wild quest to prove his love.
It could be Neville. He is often the underdog in the Harry Potter series. Maybe he discovers the spanking potion while tending to his plants in the greenhouse. He doesn't realize what it is at first and accidentally sets off a chain of events that involve the potion's strange effects on the students and teachers around him. Since Neville has a kind heart, he would then try to find a way to reverse the effects, which could make for an interesting story.
Maybe Princess Bubblegum creates a love potion to make Marceline fall in love with her, but things go wrong and it makes all the Candy Kingdom citizens fall in love with each other in a chaotic way. This could lead to a lot of humorous and heart - warming moments as they all try to sort out their new feelings.
One possible main character could be a brave adventurer. They come across the bittersweet potion during their travels and find that it has the power to heal their wounded heart from past losses, but also makes them vulnerable in new ways. Another could be a witch who is known for her potions. She creates the bittersweet potion but then realizes it has a mind of its own and starts to control the lives of those who drink it. She then has to find a way to reverse its effects.
A Slytherin prefect could be the main character. They are used to being in a position of authority and responsibility. When the potion turns all the Slytherin males into females, they take charge. They have to keep the Slytherin house in order, deal with the reactions from other houses, and also find a way to reverse the potion's effects while still maintaining the reputation of the Slytherin house.
Maybe it could be the Duke of Nuts. He might be jealous of Breezy's popularity or her special abilities. So he tries to frame her for something she didn't do in the Candy Kingdom, causing chaos for her and making her have to prove her innocence.
The Ice King could be a great antagonist. He might be jealous of the Water Princess's power over water and try to freeze all the water in her domain. His craziness and his desire for power could drive him to do this.
Finn and Jake would be obvious choices. Their courage and friendship could be put to the test in the harsh post - war world. They might be on a mission to help the survivors or to uncover the secrets of the war.
Since I don't know the specific 'love potion novel', I can't say for sure. But usually, there might be a character who creates or uses the love potion. There could also be the characters who are affected by the potion, like the ones who fall in love because of it.
I don't know the specific main characters in 'the love potion novel' as I haven't read it. But it could have a young protagonist, perhaps a girl or a boy, who is either trying to use the love potion for a good cause or trying to stop its misuse.
A new, original character could be the main character in a Beyblade time travel fanfic. This character could be a young up - and - coming Beyblader who has a natural talent but is still learning. Time - traveling would be a huge adventure for them, as they would be exposed to different Beyblading cultures and techniques. They could grow and develop their skills in unexpected ways during their time - traveling journey and become a legend in the Beyblading world.