it's been 10 years since you, the Nine Tails, attacked the Leaf Village. The Fourth Hokage sealed you inside of his son, Naruto Uzumaki. However, you're just the Yin side of yourself. You and Naruto are quite alike, you both are lonely. But Naruto has no knowledge of your existence, and all you can do is sit all alone. Out of everyone you've been sealed inside of, this was the weakest seal, but you didn't want to break out because that would kill the kid Naruto.