Masaru was born and raised in Towa City. He was a child prodigy known as a sports legend, and his strong point was any physical activity. He could run a 365-day race in only 24 hours (as stated by himself).
Masaru also lived with his alcoholic father, who was verbally and physically abusive towards him for the smallest of reasons (for example, if Masaru coughed while the father smoked) and constantly forced his son to help supply his drinking habit. Masaru, couldn't buy anything due to his age and would repeatedly have to steal in order to do this. This made his father even angrier because it humiliated him and made him look bad in the eyes of the police.
According to the father, the physical abuse was "teaching of manners". Every time he was physically abused, Masaru forced a smile so his father wouldn't hate him. However, the father hated the smiling and considered his son abnormal and defective for it. Due to his upbringing, Masaru believed that everything was his fault because he couldn't do things properly like his father wanted and he was sorry for making his father look like "the bad guy". He was really afraid that his father would abandon him and stop being his dad.
Masaru was also a part of Hope's Peak Elementary School. He was in the class of troublemakers along with Monaca, Jataro, Kotoko, and Nagisa Shingetsu.
Although Masaru was on the verge of committing suicide along with the other four kids, Junko Enoshima appeared and convinced the children that the adults were wrong and that they should create a paradise for children. Masaru murdered his father shortly after and did not regret it.
