|Age||Prologue: 8, Following chapters: 10|
|Birthdate||June 9, 2001|
|Occupation||Elementary school student|
|Model||Quentin de Gruttola|
|Voice||Max Renaudin Pratt (child)|
|“|| It doesn't matter where we live, as long as we're together.
Shaun appears disconnected from his father and other people in general. He seems melancholy, preferring to watch TV rather than talk to or play with his father. He also does not take the initiative to play on the playground, whether it be playing by himself, with other children, or with his father. If Ethan instigates the playtime, Shaun will likely join in and enjoy himself. Shaun frequently mentions his brother Jason, and how he wishes his life was still the same as it was when Jason was alive.
During the chapters The Park and Where's Shaun?, Shaun is kidnapped by the Origami Killer and Ethan Mars is left to find him. He discovers that the killer has sent him a cellphone and five origami figures. Each figure reveals a different address, and each represents a trial, a task that entails pain, risk, and/or sacrifice. If the trial is completed successfully, the video card unlocked will grant Ethan a live feed video of Shaun and will unlock letters in a hangman-style game. (Completing all trials unlocks the full address.)
Shaun is rescued in the final chapter of Heavy Rain, after Ethan, Madison, or Norman find the Origami Killer. They rescue Shaun out of the well he was imprisoned in, and after a brief CPR session, Shaun awakes.
- The Mall
- Father and Son
- The Park
- The Motel (Shown in the phone)
- The Bear (Shown in the phone, Determinant)
- The Butterfly (Shown in the phone, Determinant)
- The Lizard (Shown in the phone, Determinant)
- The Shark (Shown in the phone, Determinant)
- The Old Warehouse (Can be killed)
- Epilogue - Ethan's Grave
- Epilogue - A New Start
- Epilogue - A New Life
- Epilogue - Innocent
- Shaun is the only character whose modeling is not consistent throughout the game, appearing different in-game aesthetically than in the loading screen and most prominently in the chapter select menu.
- In the chapter Father and Son, if the player goes up to Shaun's room and looks on his desk, there will be a drawing of Jason's death drawn by Shaun.
- Shaun is approximately 4'7" (1.40 meters) tall.
- The Mall is the only chapter in which Shaun doesn't say anything at all (except the chapters in which he is dead).
- Strangely, in the promo art, the "Missing: Shaun Mars" article is shown to picture Shaun's brother, Jason when it is actually supposed to picture Shaun. It is unknown if this was simply because the brothers' roles were switched before release or otherwise.
- Shaun is the only Origami Killer victim who can be saved.
- If Shaun Mars dies in The Old Warehouse, it will be impossible to get a good ending for any of the characters (with the arguable exception of Scott, whose "good" ending would be considered the worst in the context of the story).
- Ethan Mars will always commit suicide if Shaun dies.