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 Zorndyke's 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 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)