Lopez is a robotic mechanic built by Sarge sometime before Season 1 began. Lopez's first prominent role was the maintenance of the Warthog the Red Team received from Command. He helped introduce it and repaired it when it was destroyed by Sheila. Sarge favored Lopez over the Red Team most of the time, mainly because Lopez was programmed to hate Grif. Donut later brought a voice card from command that could be put into Lopez, so he could speak. However, despite Simmons's warnings, Sarge failed to properly ground himself while installing it, and damaged the voice card. As a result, Lopez could only speak in Spanish.
At first, Lopez simply served Sarge, but Lopez developed a personality of his own over time. He soon became a sarcastic, smart-aleck robot, especially after he joined O'Malley. By Reconstruction, Lopez had become incredibly frustrated by everyone's inability to understand his Spanish.
Even though he knew the Red Team could not understand him, Lopez constantly voiced his thoughts and "experiences" to them, which ended up annoying some members on the Red Team. Sarge often made completely wrong assumptions about what Lopez is saying, generally believing that Lopez agrees with him or is making a suggestion that is the complete opposite of what Lopez was actually saying. However, in Relocation Part 3, Sarge fully understands Lopez. It would also appear as though Sarge was using him as some sort of security while he resided in Blood Gulch in Reconstruction, and has seemed to retake that position in Relocated.