Well, obviously the two main characters are the Doctor and Captain Jack. The Doctor, a time - traveling alien with a vast intellect and a love for saving the universe. And Captain Jack, a charming, immortal time agent who has his own set of skills and a rather flamboyant personality.
One idea could be a time - traveling adventure where the Doctor and Captain Jack team up to stop an alien threat that is disrupting the space - time continuum. They could visit different historical periods and use their unique skills to outwit the enemy.
Well, clearly the main characters are Jack and Johnny. They are the focus of the fanfic.
There could be a lot of banter between them. Jack might make jokes about the Doctor's sometimes stuffy and proper behavior, while the Doctor would roll his eyes at Jack's more flirtatious and carefree attitude. But in serious situations, they would support each other. For instance, when facing a powerful enemy, the Doctor would use his strategic thinking and Jack would use his combat skills and charm to get out of difficult situations.
There could also be new characters created specifically for the fanfic. For example, a mysterious girl who has the power to communicate with animals and teams up with Captain Underpants and the boys to stop an evil scientist from mutating all the pets in the town. This new character can add a fresh dynamic to the typical Captain Underpants story.
Since I don't know the exact 'captain cloud fanfic', I can only guess. Captain Cloud would likely be one of the main characters. There might also be sidekicks or companions like a little cloud sprite that helps him or a wise old cloud hermit who gives him advice.
In Captain Jack Sparrow fanfiction, Jack is a top character, no doubt. However, characters like Tia Dalma are also excellent. Her voodoo powers and mysterious nature make for great plot twists. Another good one is Will Turner. When he's included in fanfiction, his sense of honor and sword - fighting skills can lead to some great action - packed scenes, especially when paired with Jack's more roguish behavior.
Obviously, the main characters are the Doctor and Rose. They are the ones who go into the wrong Tardis.
In some Captain's Hart fanfic, there could be antagonists that oppose Captain Hart. These characters are crucial as they create conflict and drive the plot forward. They could be rival captains or evil entities from other galaxies.
In most sakura anbu captain fanfics, Sakura is the main focus as she embarks on her Anbu journey. The Anbu captain, who is often a mysterious and powerful figure, is another main character. They are usually joined by other ninja comrades from their village who add more depth to the story with their different skills and personalities.
Aside from Yama and Harlock, there could be other characters like Kei Yuki. She might be involved in some fanfics, especially those that explore the more complex relationships within the crew. Also, the villains or antagonists in the Captain Harlock universe can be important characters in fanfics. They create the conflict that Yama and the rest of the crew have to overcome.