Ahem's daughter, the water spirit. Centuries ago she was engaged to Ongo until he became evil and they were split up. Then in 2009,Rongo spotted Ongo in Tokyo with Mii. Believing Mii had taken the form of a 10 year old and taken Ongo for herself,Rongo took over the body of Natsumi's friend Nami,and tried to remove her skull necklace only to receive an electric shock.

Then upon doing a dance, Rongo showed herself and attacked Natsumi with water, and when Ongo showed up, Rongo was horrified that he didn't remember her. So Rongo challenged Natsumi to a duel and, after almost shooting a fighter jet down with jets of water, Natsumi tried to stop Rongo, but due to interference by Ongo, Rongo was transformed into a smaller child version of herself. Only then did Ongo remember her. Having lost her powers, Rongo requested to stay with Natsumi.

When Ongo became evil and giant again, Rongo tried to stop him, but eventually she left it up to Mii. Rongo's powers include being able to control all forms of water.