Katsuma Nonaka

An old friend of Hayami when he was in the Naval Academy Kastuma was a sort of upperclassmen that worked with him on the Grampus. He went to the North Pole with Hayami on some sort of secret mission but was attacked and rescued his friend before they were engaged by Zorndykes fleet. He was thought to be dead and Hayami blamed himself for it. 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 Zorndykes hybrids. Despite this he still functions as a sort of living map that the No. 6 wishes to use to bypass Zorndykes fortress. Until then he has been put into a metal waterfilled capsule to survive in. Source: Ticktank Wordpress