Valkyrie Drive: Mermaid

Season 1

Tokonome Mamori is transferred to Mermaid - an artificial quarantine island for people with superpowers. 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.