 Summer Glau as River Tam — smuggled onto the ship by her brother . A highly intelligent , compassionate and intuitive child prodigy . Experiments and invasive brain surgery at an Alliance secret facility left her delusional , paranoid , and at times violent , though her uncanny ability to seemingly sense things before they happen leaves questions as to where the delusions end and reality begins for her . The experiments seemed to have made her a psychic . The experiments also gave her with a seemingly innate ability in hand @-@ to @-@ hand combat , and she is capable of killing or incapacitating several stronger opponents with ease . She gets frequent fits of anxiety and experiences post @-@ traumatic flashbacks of her time in the Alliance facility . Her mental instability and uncanny abilities , paired with several erratic and violent acts , are a recurring source of fear and doubt among the crew , especially with Jayne , whom she once slashed with a knife . Jayne very frequently requests that River and Simon be taken off the ship . 
