Ron Weasley could also be a main character. His loyalty to Harry would make him involved in the story. He might be there to support Harry, or perhaps even be a bit jealous of Harry's new ability at first but then come to terms with it and help Harry use it to their advantage in their adventures or against their enemies.
Obviously, Harry Potter is one of the main characters since it's in the title. He is the one with the animagus ability in this fanfiction.
Obviously, Harry Potter would be one of the main characters. Since it's about him being an animagus.
Minerva McGonagall is also a favorite. Her Animagus form of a cat gives a lot of room for creativity. Fanfictions might show her observing Hogwarts in her cat form, seeing things that she wouldn't as a human, and using that knowledge to protect the school and its students.
Dumbledore could be his mentor. He has vast knowledge and experience, and he would likely see the potential in Harry being an Animagus. He could guide Harry through the difficult process of learning to control his transformation and use his Animagus form for good.
Well, Harry is central of course. But in many fanfictions, Sirius Black might also be an important character. Since he is an animagus himself, he could be a mentor to Harry in his dog form. Then there are the villains. Voldemort or his Death Eaters could be involved as Harry's dog animagus form might be used to thwart their plans. And Neville could also be in the story, perhaps being a bit confused or intrigued by Harry's new form.
Ron could also play an important role. He might be initially scared or confused by Harry's Animagus form but later becomes used to it. Ron could use his family's knowledge of magic, especially things related to magical creatures, to help Harry adapt. For instance, his family's old tales about Animagi could provide valuable insights.
Hermione Granger is often a popular choice. Fans like to imagine her as a multi animagus, perhaps as a fox for her intelligence and a deer for her grace. It gives her more depth and abilities in the fan - created stories.
Crookshanks is often a popular choice. In fanfiction, he might be given even more powers or a deeper backstory as a cat Animagus. Writers might explore his connection to Hermione in a more magical way, like he was sent to protect her all along as an Animagus.
A Basilisk Animagus could interact with Harry by initially being seen as a threat. Harry, having faced a Basilisk before, would be on high alert. But if the Basilisk Animagus was on the side of good, they could prove their worth by helping Harry fight against Voldemort. For instance, they could use their snake - like senses to detect Death Eaters approaching.
Obviously, Harry himself would be the main character. His friends like Ron and Hermione are also likely to be involved. They would probably try to support Harry or figure out what's going on with him.