Almost everyone knows Doctor Sin's personality. He's been such a larger than life character in the stories of human history that few people don't have the basic idea of how he acts. He's almost a cliche by himself. The erudite anachronism that's always ready with a koan or saying. The master of Western technologies and Eastern philosophy that can calmly torture you to death as if he was discussing tea.
Doctor Sin has come to regret allowing Sax Rohmer to record a "loose adaptation" of real life adventurer Denis Smith's adventures against him. It has resulted in his presence becoming far less feared than he ever imagined. A part of him is aware that his infamy is blunted by the fact he's been thwarted on so many occasions.
While Doctor Sin pretends to possess infinite patience and the calm of a Buddah, he's actually quite tempermental. A vengeful whirlwind of hatred and vendettas that take root in the back of his consciousness before growing ever more intense in their strength. The Doctor does not forgive the slightest affront to his dignity nor does he forget.
A part of this has to do with the fact that the Doctor defines himself by the respect he recieves. His childhood spent in the courts of China, not the Forbidden City since he had no desire to surrender his manhood but in the service of the state, was constantly a game of fighting from poverty up the ladders of respect in the exam system. The son of peasants, he has always held grandeur as the only real meaning in life.
One could probably psychoanalyze Western Imperialism, the humiliation of the Chinese Empire, the Opium Wars, the Boxer Rebellion, and the subsequent revolutions in order to talk about why Doctor Sin is so obsessed with destroying the West but that would be to underestimate what his own consciousness has crystalized into. Doctor Sin no longer blames the West for the destruction of his country. His anger at the Chinese people for their failings is almost equal to his loathing for the West for taking advantage of it. Betrayed on all sides, only his family had any real meaning for him.
....even that turned against him.
Doctor Sin's greatest secret is the fact that he is not a man whom holds any form of power to be sacred. Science, magic, alien technology, demons, crime, politics, corporations are all different ways of exercising the same fundamental views. His organization has no name since that would define it (though the Raven referred to as the Si-Fan because Rhommer felt it needed a name). Doctor Sin controls very little of his organization directly but has influence or the direct terror of countless satellite group's leaders. Everything from Japanese ninjas to Indian Thuggee to the American Skull and Bones society.
Most of Doctor Sin's time is spent studying over new developments in the world. He specializes in very few things but has used chemicals, genetics, and Chinese magic to increase his mind's capacity to grasp information. Anything could be a possible use to him and that is why he is far greater in mind than even August Roman the American King of Crime. The Doctor sleeps about four hours a week and even that is something that he often feels is too much.
Doctor Sin, in order to justify his selfish need for power, is a man that certainly talks a good game. In WW2, he blackmailed the heroes of the Age into helping him on several occasions by pretending that he wanted nothing more than world peace and an end to Japanese imperialism. It was only after the war that the communists discovered that the Manchu occupation had included many of Doctor Sin's allies (the Doctor playing all sides against one another). Doctor Sin will talk about the environment, the threat of nuclear holocaust (that one is no longer quite the good copy it used to be), or alien threats to justify his world domination but one should not take any of his words seriously. He'll share his harvested technology with his slaves but the Doctor's genorosity is limited to when he's victorious.
As a note, Doctor Sin is also supremely self gratifying. In order to make sure that his enemies do not necessary try to kill him or to preserve his sense of invincibility, he does his best to flatter them and hand out an olive branch. Even if the player characters utterly destroy his operation, he's likely to come up with a story that states this all fell into his plans somehow or that the world will suffer terribly for their foolishness. A few have accepted Doctor Sin's bribes over the years or been coerced into helping him. The dreaded man thinks that pretending he's honorable serves a far better purpose than being uniformly evil. He also hesitates to simply call a hit out on enemies but this is more the fact that his need for revenge requires it to be face to face.
While the Raven's feud with Doctor Sin is the most legendary, there are a few other heroes that the man hates almost as much. The Atom Family frequently clashed with him and destroyed his plots for world domination (the fact that Max Atom as a boy was able to thwart his efforts is particularly galling to the man). He respected the Atoms as scientists but his desire to destroy them and take their knowledge is almost as great as any other foe in his life.
With the death of Jasmine, Doctor Sin is starting to consider remarriage at this point and siring a new family. While now extremely cynical about his offspring, he's not so stupid as to think Callie Summers will ever willingly serve him. At most, she'll be his assassin to destroy her father than wed one of his lieutenants to bear an heir. Doctor Sin's last marriage attempt was thwarted in Freedom City's China town by a local tong and an unnamed trucker in yet another humiliating defeat that cost him several elemental champions...
China's communist government, questionable as it is, would probably embrace any hero that finally put an end to Doctor Sin more than any other nation. In other nations, Doctor Sin is oddly treatd as less of a threat than Overshadow.