Types of Secret Robots
Secret Robot Spy
Due to their myriad of advantages over their human or natural counterparts robots are prime candidates for being spies. Oftentimes robot spies are designed to not look like robots and their robotics are concealed over a well designed body to make them look human or animal-like.
Forgot They Were a Robot
Robots are not perfect, and occasionally their memories will become corrupted due to a major event, mechanical error, severe damage, or traumatic experience. These sorts of major events may result in a robot forgetting that they are a robot and thinking that they are sometime else, usually a human.
Another variety of this trope involves a robot purposefully erasing their memory in order to think they are truly human. However, for the purposes of the story players restore these deleted memories which result in the character realizing they are a robot.
Secret Weapon of Mass Destruction
Because the weaponization options for robots are endless they are prime candidates for being mega-weapons. In order to keep this fact secret villains or third parties will occasionally conceal the destructive potential of their robot based weapons by making them look like innocent humans or animals. Oftentimes these destructive properties are revealed during scenes in which robotic characters must protect story important characters during pivotal action scenes
Dramatically Revealed to be a Robot
Sometimes the reasons as to why a character is secretly robot are never given, but the character being revealed as a robot is still a major plot point in the games they appear in.