UFO Ultramaiden Valkyrie.

UFO Ultramaiden Valkyrie, aka UFO Princess Valkyrie, is an anime series that follows the story of Kazuto Tokino and the alien Princess, Valkyrie, who gives half her soul to Kazuto to save him after accidentally crashing her spaceship into Kazuto's family owned public bathhouse and nearly killing him. The splitting up of her soul turned her into a child, but whenever she kisses Kazuto, Valkyrie returns to her adult form for a limited time, and regains the use of her special powers.