Real Name: Arthur King (groan)
First Appearance: Justice League of America #94
Modus Operandi: Master Archer, Assassin for Hire
Powers/Equipment: Trick Arrows, jetpack hidden in quiver, archer on par with Green Arrow and family.
Backstory: Merlyn first appeared when he challenged and defeated the entire Justice League (off panel, don't ask) in an attempt to fulfill a contract taken out on Batman's life. He was narrowly defeated when the killing arrow was deflected by fellow archer Green Arrow. Shamed by his defeat and unable to return the the League of Assassins, Merlyn became a freelancer.
He did join up with the occasional villain group, including Queen Bee's gang and the mysterious "100," but kept a fairly low profile until the events of Identity Crisis. He was one of the first to realize what Sue Dibney's death would do to the super-villain community, but managed to wiggle out of any consequences due to a connection to Amanda Waller and her government sponsored Suicide Squad.
Merlyn most recently appeared in the pages of Green Arrow vol. 3, just before the "One Year Later" event. As he and Green Arrow battled, Dr. Light set off the explosives they'd both hidden previously. The resulting blast destroyed much of Star City and left Oliver Queen in critical condition. Queen survived his injuries, but as of yet the two have not had a final showdown to settle the score.