Shaun Mars is the brother of Jason Mars in Heavy Rain. Following Jason's death, Shaun's parents get divorced and receive joint custody over him. He also becomes disconnected from other people, especially his father, Ethan. He prefers to watch TV rather than have conversations or play games with him. Shaun repeatedly talks about his brother Jason, and how he would love to have his old life back, when Jason was still alive.
In the early stages of the game, the player (Ethan) is given the opportunity to stick to a timed schedule that will make Shaun a little more pro-active; these tasks include helping with homework, snack time, dinner and bed time. This will earn the player the Good Father trophy. However, Ethan can also ignore Shaun's needs entirely.
While Shaun is staying with his father, he is kidnapped by the Origami Killer who forces Ethan to go through a set of extreme trials in order to save him. It is later revealed that Shaun is being held at an old abandoned warehouse in a well that is gradually filling up with rain water, where he will drown if none of the main characters discover his location. Shaun is saved if at least one of the main characters get to him in time and wins the battle against Scott Shelby.
Both Max Renaudin and Quentin de Gruttola are credited as Shaun Mars.
Interestingly, Shaun Mars is the only character in the game that does not have consistent modelling. His appearance differs slightly between the game, the loading screen, and most noticeably in the chapter select screen where his eyes appear as light green rather than brown.