River Tam was always an extraordinarily gifted child, who excelled at everything she did. There was nothing that didn't come as easily to her as breathing did to everyone else, be it something as intellectually complex as astrophysics or something as physically challenging as dance.
Her parents enrolled her in the most challenging school program they could find - an institution known simply as The Academy. The exact goal of this institution is unknown, but they appear to be training assassains, putting the strongest, brightest minds of the world through extensive testing and complicated surgeries in order to forge the perfect weapons. Though letters were guarded carefully to keep the weapons in the institution, River managed to send her brother, Simon a coded letter indicating that she was in trouble. With a lot of planning and exchanging of money, Simon sacrificed a career in medicine in order to break River out of the institution and the two set off as fugitives into the unknown, on the run from the Alliance government.
They joined up with a smuggler's vessel on the Firefly class transport ship Serenity, intending only to stay with them as far as their next stop. But when the Alliance caught up with the ship, a confrontation resulted in Captain Mal Reynolds making the decision to run from the Alliance and take River and Simon under his care.
River's experience with the Academy causes her to be quite unstable. She is prone to strange moments of uncertainty, during which she may perform actions and be unclear of the reason for them. She has some difficulty communicating and expressing herself. And she feels everything, all emotions. She cannot repress them. Her intelligence also gives her a strange insight to the world around her that borders on psychic. In spite of this, she is known to have moments of lucidity, where she can adequately express herself and her needs. Her condition improves with each day that passes.