Obviously, Harry is one of the main characters as it's in the title. He's the android around which the story is likely centered.
Perhaps there's also a group of people who either support or oppose Harry's existence. They could be activists, other scientists, or just regular people. Their interactions with Harry would also be crucial in shaping the story.
There could be other human characters as well. Maybe there's a scientist who created Harry, and they have a complex relationship. The scientist might be constantly trying to improve Harry or dealing with the consequences of creating an intelligent android.
Android 21 and Goku are the main characters. Android 21 with her android intelligence and power is a key figure. Goku, with his super Saiyan forms and his indomitable spirit, is equally important. Their personalities are so different - Goku is more straightforward and pure - hearted, while Android 21 has a more complex and sometimes morally ambiguous nature. These differences create great potential for conflict and growth within the fanfic.
Obviously, Harry Potter and Android 18 would be main characters. Harry would bring his wizarding knowledge and skills. Android 18 would contribute her strength and fighting abilities.
In addition to Naruto and Android 17, there could be other characters involved too. For instance, Sasuke might make an appearance. He could either be in on Naruto's prank or be a bystander who gets caught up in the chaos. Also, some of Android 17's friends or allies from the Dragon Ball universe could potentially show up to see what's going on with him.
In some fanfics, other characters might also play a role. For example, Krillin, who was once in a relationship with Android 18. His presence could add an interesting dynamic, like jealousy or a sense of competition, to the relationship between Goku and Android 18.
There could be a group of characters in the 'android love story'. It might include not only androids but also humans who are either for or against the idea of androids having relationships. These different characters' views and actions would shape the story, and among them, there would be one or two main androids or humans whose relationship is the focus of the story.
In 'Harry and Teddy fanfic', the two main characters are Harry and Teddy. Harry is the one who has been through so much in the wizarding world. He has a sense of responsibility towards Teddy. Teddy is a character full of possibilities. He's the product of a tragic event (his parents' death), and Harry's presence in his life is like a guiding light. Their personalities, Harry's wisdom and Teddy's eagerness to learn, are often explored in these fanfics, which makes for very engaging stories.
In some fanfics, there could be magical animals other than crows. For instance, a wise old owl that serves as a mentor to Harry. The owl could give Harry advice on how to deal with various situations, such as when he is facing a difficult choice or when he is in danger. These additional characters add more depth and variety to the fanfic stories.
Then there might be some souls that Harry has a special connection with. These souls could be recurring characters, each with their own backstory and personality, and their interactions with Harry would drive the story forward in different ways.
Harry is the central figure. But there could also be other students who become aware of Harry's situation. For example, Hermione might be a key character. She could be researching ways to either help Harry or deal with the consequences of his transformation. And then there could be a new character, perhaps a magical cat - keeper in the Forbidden Forest who has some knowledge about cat - like magic and can offer assistance to Harry.
There might also be other Autobots such as Bumblebee. Bumblebee could be Harry's friend, always there to help him out in tight situations. Maybe there's a human - Autobot liaison character who helps Harry understand the Autobots better. And let's not forget, there could be some antagonists from either the wizarding world or the Decepticons' side who play important roles in driving the story forward.