Amagi

Amagi is a battlecruiser of Sakura Empire. She was introduced in episode 10.

She's also one of the main protagonists during an event in the mobile game: Crimson Echoes.

She's not a sister (by class) of Kaga and Akagi, but she acts as one for them, usually stopping their fights and punishing them for that. After the Sakura Empire signs a treaty where they had to stick to some limitations with heavy battleships, she was 'scrapped' (no one knows what happened to her, but she ended up dying).

As a ship, she was never completed, and would be changed to an aircraft carrier, but and earthquake damaged her and she was never completed. Her parts were used in Kaga to finish her as an Aircraft Carrier. Her bad health may be due to this fact.