This is my concept for what I think would happen if they swapped roles/classes. It's in the early stages, but I think this covers most of everything. (I think you can tell which characters I don't know much about lol)
Lady Cherri is Princess Apo's sun knight. Cherri is loyal to Apo and would give her life to protect her. She is rather serious, but knows when to have fun. Her knight ability is fire that trails the swing of her sword.
Apo is the princess of Soluna. Apo's mother, Queen Pearl, sent Apo away from their kingdom since it was under attack. She told Apo that she has Star Power that she can unlock. Apo is annoyed at how serious Cherri can be and tries to have her calm down.
Owain is a rogue who used to dream of becoming a knight, but saw how miserable most knights were, so went against that. He wants to learn how to protect himself more, though.
Kitty used to be a cat and was transformed by her owner. They killed him and ran away. He stumbled across Red Kingdom and chose to become a knight. She has some abilities of a cat, and her knight ability is that their daggers are equivalent of a larger claw swipe.
Scott was requested to be sent to Blue Kingdom due to lack of Knights. His mother was dead and later on his father dies too. He swears to protect the kingdom with his life. His knight ability is that the swing of his sword gives off a poison and blindness effect for 10 seconds.
Graecie is a light mage who has lived at the Blue kingdom for a very long time. She would sometimes see young Katie and Nom play around the Kingdom and would use her magic to entertain them.
Nom was a mischievous yet responsible kid growing up. One day Katie made a horrible mistake and Nom decided to take the blame so she wouldn't get hurt. A bounty was put on him, so he ran away to the forest. A group was sent after him, but knight 4c killed the other knights and saved Nom since he knows he did nothing wrong. Even with him being a rogue, he can't just flee to Red Kingdom to be with Katie that easily. He still feels hurt from her just leaving him, he might actually end up being killed if he tries (by either Blue, Red, or both idk), and he still really loves Blue Kingdom since it is where they grew up.
Katie was kind, but a risk taker. One day she snuck into the castle and accidentally set the library on fire. Luckily the fire was put out before the castle burnt down, but there was still major damage to part of the castle and it was taken as a threat to the monarch. Nom took the blame to protect her, but she felt so guilty that she ran away to Red Kingdom.
Knight 4c was sent in a group to claim bounty on Nom for crimes against the kingdom. 4c, however, saw what happened and saw it was Katie who ran out. He understood that it was an accident. 4c killed the other knights to protect Nom and brought him back to the kingdom under his protection. He's an oathbreaker. He has the bouncy-ness of a slime, and his knight ability is that his axe can swing and hit anyone in range.
Lady Eloise is newly knighted due to her parents making her wait awhile. She has been training for most her life. She learned a good bit from 4c. Her knight ability is that she has a spear that can send her flying in the air, and since she's a swan she can just float back down.
Bek is an air mage who grew up in Red Kingdom. She trained really hard daily. When her kingdom was destroyed, she felt like she failed them.
Frogue used to be a rogue when human, but he was turned into a frog to teach him a lesson. He chose that instead of being a rogue, he would become a water mage. (Mostly since frogs love water lol)
Shan is a health mage. Her home was destroyed and she ran away to Blue Kingdom for shelter. She wishes to help prevent anything like that happening again, so she is constantly trying to make sure everyone is safe and healthy.
Mae grew up alongside Nom and Katie, yet they never realized her. She grew up training herself and sometimes having a man she considered family help give her tips. He had to leave which made her upset, but she promised to become knighted, so she did. Her knight ability is that she can shoot 5 arrows at once that give blindness for 5 seconds.
Sausage has always been cast out. He liked to break the rules and do what benefited him. When growing up, he tried to be a mage, but they wouldn't let him go near the dark magic which he longed to use. So he gave up and has been causing chaos in different ways.
Cam's parents were not that happy with Cam choosing Necromancy as his mage class. Cam also would dabble in dark magic every now and again. Once people found out, he was exiled, so he went to Red Kingdom.
Red Kingdom: Cherri-Knight, Apo- Mage?/ Knight, Katie-Rogue, Bek-Air Mage, Cam- Necromancy Mage, Sausage- Rogue, Kitty- Knight, Frogue- Water Mage.
Blue Kingdom: Owain- Rogue, Graecie- Light Mage, Scott- Knight, Nom- Rogue, 4c- Knight, Shan- Health Mage, Eloise- Knight, Mae- Knight