Definition: Characters that use immoral means to achieve a beneficent goal
What You Need To Know: Dark Heroes are basically good guys who use deceit to save the day. They are very intelligent, and incredibly skilled at espionage and politics. They know how the game is played, and exactly how to cause it to die from the inside out.
Take note that Dark Heroes are not the same as Anti-Heroes. Anti-Heroes are protagonists devoid of heroic virtues. They are willing to commit any crime, including murder, to achieve their goals. They act outside the system. Dark Heroes, in contrast, fight the system from within.
Dark Heroes are often labeled as villains, a role they gladly accept. Dark Heroes use their status as villains to encourage the Protagonist to defeat the real antagonist. Many times, Dark Heroes are fighting not so much a person, but a social structure. They hope to cause the current oppressive era to fall while entrusting the future to the protagonist.
The title “Demon King” is often bestowed upon a Dark Hero.
My Western Take: I love Dark Heroes. Dark Heroes aren’t used very often because they require a certain degree of set up. A notable example is Severus Snape, who as an antagonist in the Harry Potter series for nearly the entire story (spoilers). You can think of Dark Heroes as a version of the Double Agent character, but with a tad more individualism.
Special Note: Itachi Uchiha (I forgot, but some Dark Heroes are driven by love)