Calling River Tam a child prodigy may not do her justice. Everything she attempted she didn't just do well, she excelled at it. Whether she put her efforts to writing, science, history, mathematics, foreign language, art, music, or even dance she was able to perform a task as if it was designed for her. River's brother Simon Tam was gifted as well, but was nowhere near the level of genius that was his sister. At the age of three she was correcting his spelling, and not long after would find errors in their textbooks. River and Simon's background was one of privilege. Their parents were wealthy, and capable of nurturing their bright, young offspring.
A child of River's exceptional intelligence naturally gravitated towards higher education, and when looking at colleges sought out the most challenging program she could find. River was offered acceptance into a program at "The Academy", a school sponsored by the multi-planetary government known as "The Alliance". River left her family to go to attend the Academy. In the meantime her brother graduated #3 in his medical class, and was well on his way to developing a strong career as an Alliance physician, most believing that he would be capable of achieving even more.
Things for River seemed to be going well at first, her family received regular correspondence when Simon began to notice that something was wrong. Her letters began to take a strange turn, referencing family events that had never occurred, and people that they had never met. Simon had no idea what was wrong, but knew that there was a problem. It turned out that the Academy was a front for a government program which experimented on exceptionally bright youth. Under the guise of a government sponsored school, the Academy performed a wide range of barbaric psychological and medical tests on River. The extent of the tests performed on River is not fully known. River's brain was mutilated, repeatedly. They performed unnecessary brain surgery; the regions of her brain that control emotion were cut into repeatedly, disallowing her from tuning out any emotion and forcing her to feel everything that normally could be pushed out of conscious focus. She was trained to kill, her body honed to be the perfect living weapon. She had psychological triggers put into her memory so that her handlers could use visual or vocal queues to do anything from enter her into a state where she would kill or put her into an unconscious state.
Meanwhile, Simon tried to convince his parents that River's letters could only mean one thing--a cry for help. She was trying to say something to them that somebody didn't want her to say. River's parents didn't believe Simon, believing that she was simply playing a childish game. Simon spent everything he had in order to find a way to get to his sister. His quest brought him into odds with the Alliance as well as his family. His break finally came when he found someone who would be able to provide him with information that would allow him to get his sister out for a price. Simon broke into the Academy and rescued his sister, making the both of them fugitives. River was put into a cryogenic sleep by Simon while he attempted to get them to safety.
While River was asleep her brother brought her aboard the smuggling ship Serenity, piloted by captain Malcolm Reynolds along with Shepherd Book and an undercover Alliance agent. The agent had followed River and Simon with the intention to turn them in. An Alliance ship tracking Serenity alerted the crew to the presence of an Alliance member on board. At first they believed the traitor to be Simon, however it was quickly revealed that this was not the case. During a confrontation the agent was fatally wounded, and crew-member Kaylee was wounded by a gunshot. After Kaylee was stabilized Mal wanted to determine what Simon had that was so important to the Alliance, and opened the cryogenic locker that River was stored in. The shock of being awakened early startled her, and she needed to be put to sleep. River and Simon were reluctantly allowed to stay aboard the ship as long as her brother would stay as the ship's physician. With the crew routinely being involved in criminal activity it was not uncommon for a member to become wounded.
The experiments performed on River have had a definite damaging effect on her psyche. River's emotional state can change rapidly between somber, happy and energetic, hysterical fear and crying, to a "killer mode". Specific events may trigger personality changes within her including extreme stimulus, danger, or mental triggers implanted by the Alliance. She is occasionally and perhaps inexplicably prone to delusions where she may perceive items or scenarios in the real world as metaphorical counterparts, which can result in dangerous situations for those around her. Simon continues to work with River, attempting to find a combination of psychoactive medication that will help her cope. Thus far there has only been marginal progress, however as time progresses River seems to be learning to deal with her past and her abilities (although at this point her personality still remains volatile).
River's full potential may not have been explored yet. She is of genius level intelligence, and is capable of excelling at virtually any task she puts her mind to. Her studies have given her an in-depth knowledge of a variety of fields. She is a weapons expert, particularly proficient in the use of blades and handguns. She is also an expert at hand-to-hand combat, using hybrid forms of fighting that incorporate aspects of multiple martial arts as well as the grace of a trained dancer. When fighting River is capable of matching as well and defeating large numbers of opponents at once. River is capable of flying ships.
River has exhibited a number of psychic abilities. She has demonstrated mild telepathic/empathic abilities. She can read the thoughts and intentions of those in close proximity to her, allowing her to tell when people are lying and knowing what people will think or say before it happens. She is sometimes over-powered when the emotions of those around her run high. She has some connection to the dead, and was overwhelmed with the voices of the dead when on a planet where all of the inhabitants had died. River is also capable of at least mild precognition and is able to sense danger before it happens.