I'm not sure exactly who Michelle Brown is without reading the story in detail. But she is likely the victim of identity theft in this particular account.
I'm pretty sure there isn't a Michelle in the Spiderman comics. The comic characters are pretty well-defined, and Michelle isn't one of them. But you never know, there could be a new character introduced in future issues.
Michelle Brown is the victim of identity theft in this story. She is the central figure whose experience with having her identity stolen is being told.
Grace is presumably the protagonist. Since the story is named 'grace and the time machine story', she is the central figure. She may be an ordinary person at first, but through her association with the time machine, she embarks on extraordinary journeys through time. Maybe she's a curious person, always eager to explore and learn, which is why she gets involved with the time machine in the first place.
Natalia Grace is the person at the heart of this complex story. She was adopted, and then there were doubts and disputes regarding her age and behavior within her adoptive family.
The 'Michelle Story' could cover a wide range of things. Maybe it's a story of Michelle's journey to achieve her dreams. For example, if she wanted to be an artist, it could detail her struggles in learning the techniques, her search for inspiration, and her efforts to get her work recognized. It might also be a story about her relationships, like her friendship with someone special or her family ties.