The Robot Mark 5 was created by Riff specifically to fight K'Z'K.
The Mark V was armored in heat-treated, triple-folded titanium. It was armed with multiple firearms, missiles, demon-blasters, spike weaponry, a ball on a chain, a screen that rapidly displayed the holy symbol of every major religion, and the Omni-Taser Supreme. The Mark V also contained the Dimensional Flux Agitator, but as an escape mechanism for the pilot rather than an offensive weapon, as Riff wasn't willing to inflict K'Z'K on an unsuspecting random universe.
The Mark V could be run either by a pilot sitting within it or by an AI based on a See-N-Say.
During the battle against K'Z'K (see The Bug, The Witch, and the Robot (chapter)), the demon levitated an unconscious Torg in front of the robot, forcing Aylee to disable it with an EMP.
The Mark V returned in Dangerous Days Ahead, where Riff sent it against Hereti-Corp with Oasis inside. The Mark V was destroyed by a sonic blast from another robot, piloted by Killum.
Mark III and Mark IV were apparently prototypes that were destroyed during Riff's experiments.