Mars-san is a tall, indigo creature with a single large eye and a single foot, interpreted to be some sort of alien. It is found weeping near what appears to be a crashed locomotive all by itself while a depressing, piano tune plays.
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 red 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 have interpreted it as a representation of Madotsuki herself, as it is all by itself in an isolated location and weeping (although her state of mind can be vastly debated). This could also connect to the traffic accident theory: the machinery next to Mars-san and its tears could represent Madotsuki's possible remorse from losing someone close to her in some sort of accident.
- Despite not having any role in Yume Nikki whatsoever, it is one of the most popular characters in the fandom.
- Some fans consider the creature that is sometimes seen in the pond on 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 or a security system, as it talks like a robot in it.
- 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. This could mean that the character represents a repressed memory in Madotsuki's subconcious.
- In the files of the game, there is an unused door meant for its location, meaning that the underground area was meant to be expanded at one point or another.
- The name of the creator of the game, KIKIYAMA (ききやま), can be translated as "machine mountain", with "KIKI" (機器) meaning machine/equipment, and "YAMA" (山) meaning mountain/hill. This could possibly allude to Mars-san.