When Gareth was a young boy, he was good friends with Doran, spending time with the explorer while he was at the palace, though they saw less of each other as Gareth grew older. When Gareth was fifteen, King Lucan commissioned Doran to write Secrets of Deltora primarily for Gareth, so that his son would learn of Deltora as a whole.
Doran intended for Gareth to wake the remaining Dragons of Deltora and left coded messages for Gareth in Secrets of Deltora, warning the prince not to trust the chief advisor and to wear the Belt of Deltora. Doran's book, unfortunately, never made it to Gareth; Drumm, Lucan's chief advisor, convinced the king that the finished book was too valuable for the young prince to handle.
Gareth succeeded his ailing father as king of Deltora when Lucan passed away. Little is known of Gareth's reign as king of Deltora, but it is clear that he was controlled by his chief advisor like most of Deltora's rulers. Having never received Doran's book, Gareth did not know about the plight of the dragons or much about life outside Del.
Doran spoke highly of Gareth, calling him thoughtful and intelligent, though he mentioned that Gareth had lost interest in reading at some point around the age of fifteen. Like most of the kings and queens of Deltora before and after him, Gareth was manipulated from a young age by his chief advisor.
Gareth was apparently an intelligent young man who was fond of playing word games and solving riddles. Doran trusted that he would understand his complicated codes in Secrets of Deltora, but it is unknown if Gareth would have noticed them had he received the book. As the king of Deltora, Gareth would have had access to all of the land's resources and guards.