Uriel is a member of the Avalon Clan. 
The egg containing Uriel was laid in the Wyvern Clan rookery in 988 and was due to hatch in 998. Due to the Wyvern Massacre however, Princess Katharine, the Magus, and Tom were forced to take all the rookery eggs away in 994. The next year, on September 28th 995, they took the eggs to Avalon. Because of the difference in the flow of time there, Uriel did not hatch until 1058. Like most his brothers and sisters, he was named after an angel by the Princess, the Magus, and the Guardian.
Together the three humans raised the "Eggs" as their own children and Uriel grew up on Avalon, with his rookery brothers and sisters, in peace and isolation from the world.
This peace was disturbed when the Archmage attacked the island in late 1995. But the Avalon Clan defeated the Archmage and his minions (including Uriel's own rookery mother, Demona) and peace was restored for a time.
The next year, when Oberon and Titania returned to Avalon, Uriel, as well as the rest of the Clan, was nearly destroyed by Oberon, but in the end the Clan found peace with the ruler of Avalon. As a member of the Avalon Clan, Uriel became considered one of Oberon's Honor Guards.
Real World Background
Uriel was named after one of the four great Archangels of Judeo-Christian lore (the other three being Gabriel, Michael, and Raphael).