Perpetually hungry, energetic, and playful, Goku is Sanzo's constant compansion—viewing him as the "sun" around which his world revolves. He is neither a demon nor human, but is a heretic born from the Earth itself. Along with his incredible strength and speed, he uses his nyoibou (extendable staff) to fight demons in battle. Tasked by the Sanbutsushin (Three Aspects) and Kanzeon Bosatsu to accompany Sanzo on his journey, Goku is often seen trading insults with Gojyo or fighting with him over food.
The diadem that Goku wears on his head is actually a limiter to suppress his true form, the Seiten Taisei, or the Sage Equal to Heaven. When in this form, Goku is incredibly powerful and nearly unstoppable. He usually can't distinguish between friend or foe in this form, and often turns his attack toward his companions. Sanzo is capable of conjuring Goku's limiter in the event that it breaks.
Backstory: Previously, he was taken from where he dwelled on Earth into Heaven due to his potential danger. Shortly after his arrival, he was placed under the care of the god Konzen Douji, who he was drawn to because of his great beauty and golden hair that reminded Goku of the sun in the world below. Goku soon befriended the war prince Nataku as well as Konzen's friend Tenpou and his companion General Kenren. Unfortunately, Goku became embroiled in the politics of Heaven. Due to the stress of certain events, Goku's limiter broke, causing him to revert to his true form. He killed a number of gods who tried to attack him, and his three friends Konzen, Tenpou, and Kenren, took him and fled. After the deaths of Goku's companions [the circumstances of which have yet to be revealed], his memories were sealed and he was imprisoned on Earth at the base of a mountain for his crimes against Heaven. Centuries later, Genjo Sanzo was drawn to the place that Goku was imprisoned by the sound of his 'voice.' The monk reached out to Goku, and the chains that bound him and the bars that confined him vanished. Since then, Goku has always been at Sanzo's side, not realizing that he has been reunited with the caretaker that he adored so long ago, as well as the two friends [Hakkai and Gojyo] that he lost.
(Source: Saiyuki.wikia & Saiyuki Reload Graffiti booklet)
