Katsuma Nonaka

He was an upperclassman during Hayami’s years at the International Naval Academy; they were on the Grampus Development Team together. To learn about Zorndyke’s true intentions, he disobeyed orders and went to the South Pole and encountered the Zorndyke Fleet. After letting Hayami escape, he was never to be found again.

Years later he is found once again, the only man to have escaped from Zorndyke, but at a price. Now a half human, half aquatic mutant, he dwindles between humanity and becoming one of Zorndyke's hybrids. Despite this he still functions as a sort of living map, that the No. 6 wishes to use to bypass Zorndyke's fortress. Until then, he has been put into a metal, water-filled capsule to survive in.

(Source: Ticktank Wordpress)