Mike O'Donnell is a fictional character from the movie 17 Again, where he is portrayed by Matthew Perry as an adult and Zac Efron as his teenage self. The story revolves around Mike, who gets a chance to relive his high school years after magically transforming back into his 17-year-old self. This gives him the opportunity to correct past mistakes and reconnect with his family. Throughout the film, Mike learns valuable lessons about life, love, and the importance of being present for his children. His journey highlights the challenges of growing up and the significance of embracing change, making it a relatable tale for audiences of all ages.