Definitely her gymnastics coaches. They can add a lot of realism to the story as they can be the ones guiding her in her training and competitions. Also, her fellow gymnasts would be great. They can be her friends, rivals or both, creating different dynamics in the fanfic.
In a Livvy Dunne fanfic, her family members could be really important characters. Her parents, for instance, could be the ones who support her dreams from the start and deal with the pressures that come with her being in the spotlight. And her siblings could be there for her, either sharing her success or helping her through tough times. Additionally, a fictional character like a sports journalist who follows her career closely could also be an interesting addition. This journalist could provide an outside perspective on Livvy's journey and maybe even become a friend or a source of some conflict in the story.
I think some of the best characters to include in a Livvy Dunne fanfic are her fans. You could have a group of dedicated fans who form a sort of fan club. They could be the ones cheering her on from the sidelines, sending her gifts and letters, and even helping her out in some way. Another good character could be a sports psychologist. Given the high - pressure world of gymnastics, this character could help Livvy deal with stress, anxiety and confidence issues, which would add depth to the story.