Spawn

Spawn is the titular hero of the long-running comic book series of the same name as well as the star of several cartoons, video games and a live-action movie based around these stories.

Originally human Spawn was cursed into the life of a Hellspawn (akin to a revenant) and became a costumed warrior fighting against the forces of Hell and Heaven in order to protect the human world from the apocalyptic wars they were planning - one of his most famous archenemies is the Violator and the Spawn series is reputed to have helped in popularise the anti-hero as a main character in comic-books since the comics came out in a period of time when darker heroes started to emerge and gained much more publicity in comic books.

Origins
Spawn began as a mercenary named Al Simmons, who was killed after attempting to quit the life of a hired gun when his conscience caught up with him. Sent to hell for his past actions as a mercenary, he was offered a deal by one of hell's Demon Lords: if Simmons agreed to become captain of Hell's armies he would be allowed to see his wife again. Simmons agreed to the deal, but little did he realize that the Demon Lord had other plans for him. Upon returning to the land of the living Simmons found he had been cursed to be what was known as a Hellspawn and forced to serve Hell forever.

Not content to be one of hell's flunkies, Spawn instead rebelled and took to fighting against hell's legions, as well as a slew of other foes. After realizing that Heaven under the false God was just as corrupt as hell he began fighting against them too, all the while keeping the war between Heaven and Hell from tearing the Earth apart and protecting humans caught in the crossfire.