It depends on what you mean by 'real'. If you're asking if it's based on actual events, I'm not sure. Maybe it's fictional or inspired by some elements of reality.
One common theme is the power - dynamic between them. Since Mirko is a strong and experienced hero and Deku is on his journey to become a great hero, their relationship can explore how they support and learn from each other in terms of power and skills.
There might be a fanfiction where Izuku is training under Mirko. She pushes him to his limits and helps him unlock a new aspect of his quirk. This story could focus on their mentor - mentee relationship and how Izuku grows from it.
Greta is partially based on a true story. The filmmakers likely incorporated real-life elements and added fictional elements to make the story more engaging and cinematic.
There are fanfictions that explore Mirko's backstory in more detail. For example, how she became such a strong and fearless hero. It might delve into her training days, the challenges she faced, and the people who inspired her along the way. In some fanfictions, she could have had a mentor who taught her unique combat techniques that she uses in her battles now.
You can start by looking on websites like Archive of Our Own (AO3). It has a large collection of fanfiction from various fandoms, including ones for Izuku and Mirko. Just search for their names and you'll likely find many interesting stories.
One highlight is his amazing kickboxing career. His left high kick was a feared weapon. Another is his entry into the UFC which brought him international fame. He also had epic battles against top fighters.