Valkyrie Drive: Mermaid
The "beautiful girl sexy battle action" anime is set on five artificial islands. Mamori is a "mermaid" transferred to one of those islands. When Mamori is attacked, a newly transferred girl named Mirei saves her. The enemy doesn't stop, however, and the two are soon cornered. Just when they think all hope is lost, Mirei kisses Mamori, and Mamori turns into a sword. Mirei then wields the sword and launches a counterattack against their enemies.
(Summaries provided by MyAnimeList.net)