The man who grew up to be known as the Mock Mikado was born in Japan close to the end of the 19th Century to a man claiming to be an emperor of Japan. He was one of two boys born that day, leading their father to fear that with two successors, a civil war might divide the country. Instead, he ordered his loyal servant Baron Jujitso to take one of the boys to North America where he, dubbed the Mock Mikado, would grow up to follow a destiny of conquest of North America so that both boys could have their own domains to rule. Jujitso took a legion of loyal followers to Mexico where they lived and raised the Mock Mikado for years. By the winter of 1941, Jujitso became mortally ill, and on his deathbed, ordered the Mock Mikado to fulfill his destiny and invade the United States of America.
Amassing his army, the Mock Mikado crossed over the US/Mexican border in the fall of 1942. Their path of conquest was spotted by Captain America and Bucky who were both stationed in California in their civilian guises of Private Steve Rogers and Bucky Barnes. Despite their best efforts to stop the invasion force, they were captured by the Mock Mikado who decided to keep them alive long enough to witness the capture of California, the first step in his conquest of America.
The Mock Mikado attacked the set of a film studio as well as attacked a military outpost before setting up a camp and preparing an air raid on California. Although the Mock Mikado had apparently taken over the entire state, Captain America and Bucky managed to free themselves. Rallying soldiers together, Cap and Bucky lured the Mock Mikado and his men into a dead end canyon where the military gunned down his minions. The Mock Mikado's invasion fleet was also destroyed by American bombers. With his plot in shambles, the Mock Mikado was tied up by Captain America and Bucky and was left for military officials to take into custody.
His fate following the war is unknown.
The Mock Mikado was armed with a samurai sword.
The Mock Mikado rode a white horse.
There is no historical proof that the Mock Mikado was actually the son of the emperor of Japan, and any connection between the Mock Mikado and the true Japanese emperor Hirohito is speculative at best. His origins are likely the product of Imperial Japanese propaganda to create a loyal soldier, or the product of American propaganda that was common in the recounting of Captain America's adventures during the war in comic books in order to earn more support.
The term "Mikado" is Japanese for emperor.