Rantaro is the survivor of the 52nd killing game on the show Danganronpa but was made to participate again. He is the Ultimate Survivor.
After he successfully joined Danganronpa as a Killing Game participant, Rantaro's memories and past were fabricated by Team Danganronpa as follows.
Rantaro claims he comes from a very rich family and that he was raised like a celebrity. He had multiple younger sisters, most of them step sisters, and an unknown amount of half-brothers. It's implied his father is a businessman who might be a big womanizer, explaining the large amount of children. The family also had a kind maid when Rantaro was a kid, who took care of the children and was like another mother for them. For Rantaro, she was an amazing person who could do anything. The siblings sometimes had disagreements over silly things like who gets to read a picture book first with the maid.
Rantaro felt responsible for his younger sisters and wanted to help them and take care of them, to the point that he apparently learned a lot about make-up and other things that are often called feminine. He claims that he frequently helped his sisters with nail art. He was especially close with his youngest sister, who always followed him around.
Many years ago, he and his entire family went on a boating trip. It was Rantaro's first one. He liked traveling and once they reached a port, he got off the ship and went on an adventure. His youngest sister followed him off the boat without him realizing and as a result, she disappeared and was never found. Rantaro blamed himself for her disappearance, believing himself to be a terrible older brother and feared that his sisters might hate him.
However, Rantaro was determined to find his sister and with his father's permission, he traveled around the world on a boat searching for her, while simultaneously helping with his father's business in search of new markets. He believed that his sister was alive unless he was proven otherwise. The traveling was dangerous, especially when he had to cross territories. Somehow, Rantaro lost all of his 12 sisters all around the world, and his family and everyone else told him to give up. He is determined to reunite his siblings no matter how many years it would take him.
He was murdered by Tsumugi who later framed Kaede. He was later revived. He joins Kokichi and the others in rescuing Kaede, and they all reunite with Shuichi and the other survivors, joining the Brave Adventurers in the process.