Ain is the second-in-command and Vice-Admiral of the Neo Marines in the One Piece Film: Z, and a former member of the Marines.
When she was a Marine soldier, she fought with Zephyr and Binz against a powerful pirate that cut off Zephyr's arm. She and Binz were the only soldiers other than Zephyr to survive.
After the said pirate was promoted to the ranks of Shichibukai, it caused Zephyr to leave the Marines out of indignation; and in loyalty, Ain resigned her position as a Marine officer, alongside Zephyr, Binz, and Shuzo. All of them would become the founding members of the Neo Marines. She eventually fought against Shuzo for the position of second-in-command, and after a five-hour battle, when she was on the verge of losing, she strategically trapped Shuzo in the ground and threatened him with a pistol, winning the position.