backstory
Rantaro claimed he came from a very rich family and that he was raised like a celebrity, and that he has multiple younger sisters, most of them stepsisters.
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 starting to fear 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, whilst simultaneously helping with his father's business in search of new markets. He firmly believed that his sister was alive unless he was proven otherwise. The traveling was dangerous, especially when he had to cross territories.
He gained his Ultimate Adventurer title during one of his journeys through the savanna when he came across a remote village and helped the villager with their research about a rare disease that only affected the people there. He began to get famous after that, traveling the world, and eventually, people were calling him the Ultimate Adventurer.
Somehow, Rantaro lost all twelve of sisters all around the world, and everyone, including his family, told him to just give up on his search. Despite this, he was still determined to reunite with his siblings, no matter how many years it would take him.