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" Hulk because of his refusal to stop fighting.
After her defeat by the Hulk Mercy was held in an unknown military facility until she was release by General Thaddeus Ross, with the condition she be a member of the Thunderbolts. Mercy agreed since she also shared a history with Ross and had hopes of bringing her gift to The Leader and Ross himself.
During Thanos' invasion of earth with the infinity event Mercy went Rogue and decided to give her gift to the humans and invaders alike. This act caused the Thunderbolts to devise a plan to get of her. The team recruited Johnny Blaze in order to find a way to send her to Hell, since it was a place of despair in which people wanted her gift as the leader theorized. This plan failed and The Thunderbolt instead where transported to Hell, but before they found a way out they made a deal with Mephistopheles. The deal was they would help him reclaim his throne if mercy is transported to hell and they to earth. It is during this time that mercy could no longer fell the presence of Ross or The Leader on earth any more and confronted The Punisher and Elektra to interrogate them to the teams location. Mercy destroys the punisher's hand and was about to kill them when she is transported to Mephisto's realm, where she joyfully ask if hell was heaven. The team leaves Mercy stranded in Hell where she joyfully gives her gift the damned souls who seek it.
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.