Marty McFly, Jr. is the son of Marty McFly and Jennifer Parker in 2015. He is known for being weak and somewhat impressionable, being pushed around by Griff Tannen. In Back to the Future Part II, Doc Brown recruits Marty and Jennifer to go into the future to save him. In the first timeline, Marty McFly, Jr. had been arrested by being bullied into doing a crime by Griff.
When Marty and Jennifer went into the future in the second timeline, Marty dressed as Marty McFly, Jr. , and confronted Griff, stopping Jr. before the crime had even been committed.