Appa Ali Apsa once assumed human form to travel across Earth with Hal Jordan and Oliver Queen. Originally he joined the two heroes to observe mankind, but he found himself befriending both earthmen. After he used his power to save his friend Hal Jordan, the other Guardians deemed that Appa had been corrupted by emotion.
He was stripped of his immortality and returned to the Guardians original homeworld of Maltus. Appa's powers were restored by his brethren who appointed him the custodian of Oa after the first destruction of the Central Power Battery.
However, loneliness led to madness and Appa created a Mosaic world on Oa of all of the places he had visited in his earlier life. He planned to rule over this planet as a God, but was stopped by the combined powers of the Guardians and the Green Lanterns of Earth.