Ron Weasley could be in it as well. He might be the one who accidentally discovers Harry's first transformation or provides some comic relief during Harry's adventures as an animagus.
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.
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.
In many fanfictions, Sirius Black is still a very popular animagus character. His ability to transform into a dog makes for interesting interactions with magical creatures, especially those that are friendly to canines. Another popular one is Minerva McGonagall. Her animagus form of a cat allows for some really unique scenarios when she encounters magical cats or other feline - like magical creatures in fanfiction stories.
Harry Potter himself is often a main character. Since he has a connection to stags through his Patronus, it's common for fanfiction to explore his potential as a stag animagus. Another is James Potter, Harry's father, who was an animagus and took the form of a stag.
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.
One interesting plotline could be Harry's discovery of his animagus form in a unique way. Maybe he doesn't follow the traditional path of learning in Hogwarts but instead has some sort of accidental transformation that leads him to explore his new ability.
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.