Ranmaru was one of the 20 participants forced into the Death Game, he was alone, and died during the First Trial from something unknown that caused a large wound on his stomach. Before he died he describes that he had been feeling no pain, possibly indicating that he had become numb to it in the end.

Ranmaru is a casual and easy-going boy. He is generally placid in nature, although occasionally flustered, such as when Sara holds his hand. He seems to come off as nervous or awkward in certain situations. Ranmaru also tries to keep quiet and distant until provoked.

He is shown to be compassionate and expresses a want to be useful to the others.

After his death, Ranmaru's AI was uploaded into a doll that looked identical to him in order for him to help the Death Game participants during the third chapter. He was paired with Sara as her support.

From what is shown in Chapter 3 so far, he has an awkward demeanor to him when he meets Sara for the first time, and is nervous to be her pair. He's willing to work with Sara and help her solve puzzles or fight in battles. At times he appears a bit flustered by Sara, for example when she holds his hand to enter the Discussion Room, he's surprised and blames her for being so nervous when she tells him not to worry about it, else he wastes his battery power.