Ann Darnell was a life support specialist before gaining her powers.

The Hood has hired her as part of his criminal organization to take advantage of the split in the superhero community caused by the Superhuman Registration Act.

She is the sister of teammate, X-Ray

Vapor can transform her body into any form of gas, either pure elements or complex compounds. She can only return to solid form for brief periods. She is vulnerable to any chemical interaction that would affect the chemical-form she is currently in. On one occasion, the Hulk defeated her by spraying her with oxygen while she had taken the form of hydrogen, turning her into water. While generally immune to physical harm, she could still be harmed by different types of energy, such as Thor's winds and lightning, or the sonic booms the Hulk produces by clapping his hands together, and even X-Ray's radiation.