Valkyrie Drive: Mermaid

Mirei, Mamori, and Kasumi come across a girl named Nimi Minimi who, after running away from the castle when Charlotte tried to force her and her girlfriend Noe Oya into separate partnerships, somehow wound up in a giant form. Nimi is brought to Atelier Torino, where she manages to get along with everyone despite her gigantic form. The next day, as Nimi suddenly grows even larger, Mamori encourages her to go and see Noe and tell her how she feels. This naturally causes panic as she approaches the castle, with Kasumi's attempts to stop her only causing her to grow larger still. Before Charlotte can attack, Mirei and Mamori break Noe out of detention so that she can reach Nimi, allowing her to return to normal size. Akira then explains that the growth was a cause of Nimi's powers, which had simply taken a while to activate, allowing Nimi and Noe to remain partners with each other.