Multiple explanations for her origin exist, including that she is an alien, an angel, or that her powers were induced due to radiation therapy for brain cancer. She considers herself to be an "angel of mercy," killing those who do not have the strength to end their own lives through suicide. She has been unable to "help" The Hulk, because of his refusal to stop fighting.
Mercy was created by Todd McFarlane and Peter David, first appearing in The Incredible Hulk #338, December 1987.
Powers and Abilities
Capable of flight/levitation, physical intangibility, invisibility and teleportation, she can read minds and direct spears of energy that can kill.
In the 2005 video game, The Incredible Hulk: Ultimate Destruction, Mercy is one of the bosses. Her character is voiced by Vanessa Marshall. The explanation given for her abilities in the game are as a result of receiving gamma radiation for her brain tumor.