Tenchi Muyou! Ryououki

Seventeen-year-old Tenchi Masaki grew up hearing stories about how his ancestor used a sword to seal a demon inside a cave seven hundred years ago. When curiosity gets the better of him, Tenchi goes to the cave and stumbles across the sword from the legend. Thinking that the story is nothing more than a fairy tale, he removes the blade and inadvertently releases the demon, who turns out to be a space pirate named Ryouko Hakubi. Furious about being trapped for so long, she attacks Tenchi, but he is able to repel her with the sword, awakening his inner power. After seeing this, Ryouko takes an interest in her unlikely savior and decides to crash at his place.