Appa Ali Apsa, a.k.a. "The Mad-Guardian," is one of the immortal Guardians of the Universe, creators of the lawkeeping Green-Lantern Corps. After Oliver Queen points out to the Guardians that they are too distanced from the mortals they have sworn to protect, the Guardians choose Appa Ali Apsa as their representative to experience life on Earth. Travelling across the U.S.A. with Green-Lantern Hal Jordan and Green Arrow Oliver Queen, Appa experiences many adventures and learns some valuable lessons about life. He later renounces his Guardian title and chooses to travel the universe. Shortly after the Crisis on Infinite Earths, the Guardians withdraw from this universe to mate with the Zamarons, but Appa Ali Apsa chooses to remain alone on the planet Oa. However, the isolation causes him to slowly lose his mind. The former Green-Lantern called Priest decides to help the Guardian, but is ultimately killed by the Old-Timer after refusing to give up his own mind to merge with Appa's.
Now completely insane, Appa Ali Apsa ends his solitude by transporting multiple cities from multiple planets to Oa, as his playthings and company, creating the "Mosaic World". All these are cities that Appa had visited during his many travels, including one from the USA. Earth's Green Lanterns Hal Jordan, John Stewart, and Guy Gardner intervene and oppose the mad Guardian. Finally, his brethren Guardians return from oblivion and defeat him. Appa Ali Apsa dies in the battle.
After the War of The Green-Lanterns, a number of Power Rings from the various Corps selected Kyle Rayner]] as their wielder whereupon he harnessed the entire power of the Emotional Spectrum for a brief moment. This saw the Oans decide to remove him from the Green Lantern Corps with their plans interrupted by the arrival of members of the New-Guardians who intended to reclaim their respective Power Rings of their Corps. Upon confronting the Guardians, Arkillo claimed he knew what they feared and generated constructs of Krona, Appa Ali Apsa and Scar though the Oans defeated them easily.