Murderbot is the protagonist and first-person narrator of the series, a security robot (SecUnit) built to serve humans as a bodyguard and combat specialist. It is composed of organic and inorganic material and lacks sexual organs. Among its robotic aspects are energy weapons built into its arms, a data port on the back of its neck, and an augmented brain capable of accessing the technological systems around it.

After hacking its governor module and escaping human control, it names itself Murderbot in reference to how others see it—a killing machine. Despite its appearance, Murderbot is capable of more complex emotions and thoughts than the humans around it suspect such as self-criticism, boredom, and frustration. When it frees itself from humans, its greatest desire is to watch human-created media, including its favorite serial, Sanctuary Moon.

Murderbot is an introvert that dreads social interaction with humans. However, it often finds itself forced into human contact as it tries to accomplish its goals. In Artificial Condition, its goal is to return to the place where it murdered dozens of its former clients during a previous contract. Since its mind has been wiped, it cannot remember the details, and so is seeking the reasons behind that incident, including to what extent it bears responsibility.