Well, obviously Aquaman himself would be a key character. He could be a mentor to his daughter, guiding her in the use of her powers and in understanding her responsibilities. Mera, Aquaman's wife, could also be an important figure, perhaps teaching the daughter some of the more feminine aspects of Atlantean culture or how to control her water - based powers in a different way. Then there could be the daughter's friends, like other young Atlanteans who she goes on adventures with. They might have different powers or skills that complement hers.
A possible plot is that the daughter is sent on a quest to find a lost artifact that is crucial for the balance of the ocean. Along the way, she discovers secrets about her family's past that her father has been hiding from her. This could lead to some tension between them, but also help her grow as a character. She could also face challenges from other Atlantean factions who don't believe she is worthy of this task because she is still young and inexperienced.
There could be a wise old wizard who lives in a tower in Arcadia Castle. He has vast knowledge of magic and often gives advice to those in need. Another character could be a young and curious maid who always seems to stumble upon the castle's secrets while going about her daily chores.
You could include a wise old mentor figure, like an old gardener who gives Cinderfella advice. A friendly mouse that helps Cinderfella around the house. And of course, the beautiful princess who Cinderfella falls in love with at the ball.
A young apprentice wizard who is new to Potter Palace and has a lot to learn. There could also be a mysterious gardener who tends to the magical plants around the palace and knows many secrets. And a wise old owl who can talk and gives advice to the characters.
A young and ambitious sand ninja named Kaito could be a great main character. He's eager to prove himself and rise through the ranks in Sunagakure. Another could be a female ninja, Saya, who has a special jutsu related to sand manipulation that sets her apart. Or perhaps an older, more experienced ninja, Ryuji, who has seen many battles and has a lot of wisdom to offer but is also haunted by his past actions in the service of the village.
Red could be a great main character. He's one of the most iconic Pokemon trainers. You could tell the story of his journey from a different perspective, like how he got his first Pokemon and what his real motivations were at the start of his adventure.
A mysterious outsider who suddenly appears in the world of the Northern Star could be the main character. This character has a hidden past and strange powers that are gradually revealed as the story progresses. Their presence shakes up the existing order and sets in motion a series of events related to the Northern Star.
Sakura could be one of the characters in the harem. She is strong - willed and her medical ninja skills could be useful in the story. Hinata is also a likely candidate as she has always had a deep affection for Naruto. Her gentle yet powerful Byakugan abilities would add to the group dynamics.
A wind - up mouse could be a potential character. It's always scurrying around and stumbles upon the toy lemon by accident. Then there could be a wise old toy owl that gives advice to the other toys about the significance of the toy lemon. And a bouncy rubber ball that adds some fun and energy to the group of toys in the fanfic.
Well, there could be doctors like Dr. Smith, a brilliant surgeon. Nurses such as Nurse Johnson, who is very kind but strict about following procedures. There might also be patients like Mr. Brown, a friendly old man with a heart condition. And don't forget the hospital administrators like Ms. Davis who is always trying to keep the hospital running smoothly.