Mars-san is a tall, indigo creature with a single large eye and a single foot. It is found weeping near what appears to be a crashed locomotive.
Mars-san produces a remorseful three-tone chime (high b flat - e flat - f) when Madotsuki interacts with it, coupled with a small darkening of the screen. Additionally, Mars-san can be stabbed with the Knife. Instead of dying, it leaves a wound on its foot and changes the color of the eye from red to green with a blue pupil; a second stabbing will change the eye color back to red with a pink pupil.
- Some fans consider the creature that is sometimes seen in the pond on The Witch's Island to be the child of Mars-san due to their shared likeness.
- Mars-san is a significant character and a large source of speculation, being included in most theories about the game.
- In the manga, Mars-san behaves similar to a supercomputer, a robot, or a security system, as evidenced by robotic vocabulary.
- Mars-san bears a resemblance to the 1926 surrealist painting "Person Throwing a Stone at a Bird" by Joan Miró. Other characters also resemble famous surrealist paintings, such as Tokuto-kun and FACE.
- In order to reach Mars-san, you have to fall sleep in three different beds, meaning you would be dreaming in a dream in a dream.