Kyoushirou to Towa no Sora EP12 - Eternal Kuu

The Bastille releases the Absolute Angels, who are reunited with their respective partners. Kazuya's ambitions for Cielo are crushed. The destruction of the Bastille, however, costs Kū her life. Kyoshiro promises her many things to do 'the next time around' before all of her mana is drained away. The world returns to peace and Kyoshiro and Setsuna embark on a journey to find Kū's reincarnation, which they do succeed in years later —— This is shown after the credits when Kyoshiro and Setsuna find Kū.