Obviously, Bella is one of the main characters. Since it's about her having triplets, she'll be central to the story, dealing with all the new challenges.
There might also be other characters from the original source material, like Bella's friends or family members. For example, her partner might be an important character, helping her raise the triplets or having their own reactions to this big change in their lives.
Obviously, Harry Potter is one of the main characters. He's the father of the triplets. Hermione and Ron would probably also be important characters, as they are Harry's closest friends and would likely be involved in the triplets' lives, perhaps as godparents or just as trusted advisors.
One possible interesting plot could be the challenges Bella faces in raising three children at once. It might include things like dealing with their different personalities, and making sure each one gets enough attention.
The triplets themselves are usually the main characters. They could be given names like James, Lily, and Sirius as a nod to the original series' characters. Often, they'll have traits that set them apart, like one being extremely intelligent, another very brave, and the third having a heart of gold.
It could be her best friend. Her best friend might be the one who accidentally discovers Bella's secret in the fanfiction. And then they have to deal with the situation together, whether it's keeping the secret or helping Bella face whatever the secret entails.
They could be the children of Luke Skywalker. Maybe he had triplets in secret, and they inherit his strong connection to the Force. Each of them could have different personalities, like one being very brave and impulsive, one being more cautious and intellectual, and the other being empathetic and diplomatic.
Her best friend could be a main character. This friend could be the one who has always been there for Bella, through all the hardships of her past. They might try to protect Bella from the things that are resurfacing from her past, or they could be the one pushing her to face her past head - on. For example, the friend might find out some new information about Bella's past and decide that it's time for Bella to know the truth, even if it's painful.
There could be a kind teacher who tries to help Bella. This teacher might notice that something is wrong in Bella's life and tries to offer support, like giving her extra food or a listening ear.
Since I don't know the exact 'triplets secret novel', I can't say for sure. But likely the triplets themselves are main characters.
Since it's a fanfic, it might introduce new characters that are unique to this story. There could be a rival within the federal system who is jealous of Bella's new - found status with the badge. Or perhaps there's a mentor - like figure who has been in the federal service for a long time and helps Bella understand the ropes. Another possibility is that there are informants that Bella has to deal with. These informants could be from different backgrounds and have their own motives for helping or hindering Bella in her work with the badge.
The main character is always Bella in this type of fanfiction. However, new characters could be introduced as well. For instance, a blind mentor who has learned to live fully with blindness could be added. This character could guide Bella and help her accept her condition. Or there could be a villain who tries to take advantage of Bella's blindness, which would then lead to a conflict where Bella has to use her wits and other strengths to overcome the villain.