He is the current King of the Celestial World who sets up this whole tournament. As shown in the anime, God is a pervert who likes women and drinking. In a flashback, he was inspired to start the tournament, due to the actions of Ueki's adoptive mother, Haruko Ueki, who at the time was a high school student just before she died.
He started the tournament to give all beings, whether human or heavenly being, a chance to create their own futures. Although his front is a mindless idiot who likes to slack off, he actually very wise and powerful. According to a text box in the English manga, he lives in a skyscraper in the USA.
He's considered the best King of the Celestial World in history, which is a fact that he is very proud of.
Although he was pierced by Anon's Ranma, he survived and was later shown in hospital, and also managed to restore some of the years to Robert's life that Anon had used up.