Warp Darkmatter was once Buzz Lightyear's partner and best friend, but he had been secretly working for Zurg since his academy days as a double agent. He eventually began working for him full-time as "Agent Z" after faking his death. His right arm is mechanical (with various weapon attachments) and was grafted to his body after becoming Agent Z. Warp often takes on assignments for Zurg and is paid quite handsomely (he owns a moon and a summer home). Though Buzz has vowed to bring him in for treason, he retains some hope that there is still some good in him.
Warp Darkmatter has some similarities with Bucky Barnes, they both have prosthetic arms and are former partners of the main protagonist and were presumed dead. Unlike Bucky, Warp was evil to begin with.