He was a real person, but he was a court pet and not a samurai. He was a Portuguese slave bought by Nobunaga who used him to entertain guests. When Nobunaga was betrayed, and then the betrayer was defeated, Yasuke fell into the hands of Nobunaga's successor. At first they wanted to execute him because he should have died with his master, but then they decided he was actually an animal and not a person so they gave him to a Christian missionary. That is basically the entire story.