I'm not sure specifically which 'Mili' you are referring to. There could be many things named Mili. It could be a character in a book, a movie, or a song. Without more context, it's difficult to determine whose story it is based on.
It could also be a fictional story that was created by the screenwriter out of pure imagination. Sometimes, screenwriters draw inspiration from multiple sources. Maybe they combined elements from different people they met, or stories they heard, and then crafted the story of 'Mili'. So, without seeing the movie or having more details, it's really hard to tell.
Not likely. Usually, when something is based on a true story, that's made clear. But in the case of Mili, there's no indication it's rooted in reality. It's more likely a product of imagination and creativity.