The Prison is where Six is imprisoned as well as The Runaway Kid, and this is where they start their journey. Six is the main character, but during your time in the prison, you can see The Runaway Kid in a cage next to you. You can also see Six while playing the DLC as The Runaway Kid.
While she is in the prison, you can see that she doesn't mind using other kids to help herself. This is one of Six's more antagonistic aspects. The Janitor is the ruler of the prison, and is the one given the job to, well, prison them. In the prison, you can see about 7 other kids, all locked up, not even trying to escape. This is one of Six's good, not-going-to-give-up aspects. Speaking of aspects, Six and The Runaway Kid both have similar thinking processes. They both want to escape. They both do. The Prison is an important chapter in the entire game of Little Nightmares.