Hanna is a CIA research subject.
At least she started out as one, as a baby, and years later she was very much wanted back in the program.
In the months prior to her birth, Hanna's mother was an unwed pregnant girl planning to have the pregnancy terminated when she is approached by CIA operative Erik Heller and offered a chance to become part of a project, which we later learn is called Utrax. The idea of the project was to manipulate the genes of fetes from an array of young women, including the insertion of a small amount of wolf DNA, to increase the strength, endurance, and innate fighting abilities of the child.
When Hanna's mother had a change of heart about giving the new-born up to the Agency to be raised, she and Heller, who had fallen in love with her, decided to 'steal' the baby and flee, putting the entire program into jeopardy. In the escape attempt, the woman was killed but Heller and the infant managed to escape.
The powers at the Agency who had green-lit the project in the first place, decided to terminate Utrax and ordered its project lead, veteran operative Marissa Weigler, to destroy all remaining test subjects and erase all evidence of the program's existence.
Jump ahead 15 years and a now-teenage young girl named Hanna is living in the wilderness of a large forest in Poland (Finland in the movie version), raised by her adopted father Erik and trained every day to be the very best fighter and survivor she could be, all in preparation for the time when the CIA would find them.
Her skills at hunting and killing and taking care of herself in a wide range of situations, all taught by Heller but very much improved by the enhancements to her DNA, make her a very impressive weapon. She is highly intelligent and quick thinking but she is totally naive about the human interactions. Heller had tried to make her aware of what it was like 'outside', lack of actual contact and personal experience causes considerable problems when she is thrown back into society. Especially since the CIA is still constantly on the lookout for any sign of her existence.
Once her whereabouts are discovered and Weigler brought back to help in her recapture, Heller decides to let Hanna know the full truth of her origins and in so doing, he and Hanna and even Weigler will all learn that the Utrax program had been restarted and that there are several dozen teenage being indoctrinated into being perfect killing machines totally obedient to their masters.
Hanna has a new mission far beyond just survival.