Japan thought that his pacifism would keep him safe. Only to once again feel the horrors of the atom bomb, as Tokyo and other cities were destroyed in Doomsday. Barely alive and supposedly left alone to die, he nonetheless sought to rebuild with what was left of his people. And against the odds, order and Japanese virtue prevailed, saving and upholding as much of his shattered land as possible. Such was his determination that his country has once again grown strong if rather secluded.
He retains much of his self-control, discipline and peculiar manners, although years of isolation have made him a bit of a recluse (if not an outright Hikkikomori), as well as taken to wearing traditional attire more often. But time hasn't eased his anger and frustration, especially as he learned more and more about what really happened. As a result, he bears a grudge against America (whereever he is) and the Survivor-Nations that had risen from his so-called "friend's" ruins. This was also the reason why until recently and despite keeping on good terms with Korea, Socialist Siberia and the Philippines among others, he's kept largely to himself...for fear that the changed world might harm him.
UPDATES: Credit goes to for the character descriptions.
Character not mine. Kiku Honda belongs to Hidekaz Himaruya.