That reminds me, before I start to adapt the backstory I originally worked up for the concept away from Korea and to China, would you mind giving it a look-over to see if the main points are alright?
Captain Mizuhara Jin
Born in 1875, Jin was the child of Mizuhara Jinsuke, a low wage teacher, and Mizuhara Tamako, a housewife in a small village north of Kyoto. As he grew older, Jin quickly grew unhappy with their rural situation. Despite a good basic education, the lack of money prevented any chances of getting further in an intellectual career. As a result, he joined the military, in hopes of completing his education there and joining their scientific research department.
However, he soon found that his sound mind and determination opened him entirely new options once he had completed basic training, finding the structured and demanding life in the military both refreshing and rewarding -- rather than going with his original plan, he enlisted for officer training.
He took a part in the first excursions by Japan during World War I and was involved in several attacks on German outposts in East Asia. His success eventually earned him a promotion to the rank of Captain and a transfer to a less volatile region. To Korea, where he was to supress remaining resistance against the Japanese annexation and prove his administrative skill, rather than his tactical acumen.
Once there, however, he quickly fell into stagnation. Not even his marriage to a local woman named Su-Min brought any solace. Always a man of strong beliefs and integrity, he despaired over the lows some of his countrymen had fallen to, growing more and more frustrated until he began to dole out harsh punishments far beyond the norm, spiralling down just as much as his fellow soldiers were through a combination of disappointment and loathing, of himself and others.
Eventually, during the March 1st Independence Movement of 1919, when over two million rallied against the Japanese oppressors, Captain Mizuhara fell. Not to the riots, however. He was labelled a victim of the rally, but in actuality one of his subordinates gunned him down during the confusion, successfully getting rid of a hated superior.
The dark aspect of Jin's self found itself in Mikaboshi's Wicked City. It took the p'o some time to disentangle itself from the many technological horrors found there, but it eventually did. He rose as a newly born Wan Kuei, in Seoul.
Since he had had a Korean wife (and, born after his death, a child), his remains had been interred there. Despite his nationality and station, Jin had always treated the locals with respect, if not kindness, and that had found the same in return: his famous cruelty had been directed solely at his own men, after all.
More a point of curiosity despite his acts as a mortal due to being a foreigner, he was nevertheless considered to have potential and inducted into the Howl of the Devil-Tiger. His time in the Wicked City had had one upside: he didn't need much catching up to modern standards, even if the knowledge comes with horrible trauma.
Jin conducts himself as a highly disciplined man, though the time in Yomi Wan and the extensive torture performed upon him to awaken him to his proper self has left its marks. He has exchanged his military uniform for civilian attire, now preferring tailored suits and enjoying a new appreciation of creature comforts, despite his full realisation that they are but a vest to wear: a demon may be wretched, the Searing Wind must be magnificent.
He idly wonders what became of his relatives back in Japan, but has not found the opportunity (or, indeed, proper motivation) to look into things, now that he has become one of the righteous devils. The same goes for his descendants within Seoul -- he has actively avoided looking into them, having no desire to get them involved in Kuei-jin activity. After all, he knows all too well that at some point he'll have to cut his mortal ties, if he wants to complete his Dharma -- literally. He sees no reason to rush into such things or worry at them before the time comes.
Generally, Jin is a practical man of strong moral fiber and as such finds the aspect of punishing sinners the most rewarding -- if he has to become a monster himself to do so, then he had already taken the first steps towards that journey as a mortal. Half-finished things are a sign of weakness.
Edit: It's a fairly old writeup, so I'll probably rephrase quite a bit along with adapting things.