Taliesin is a Wandering Bard or better known as a Minstrel. He was first introduced in episode 5, Londinium. He played the harp for the group and informed them that the place they were hunting belongs to the Brigantes Tribe.
When at Avalon Castle, he tends to tease Epona, touching her and saying "soft and smooth"
He is the chieftain of the Brigantes tribe; the direct descendants of Pwyll that the Gael tribe broke away from. He later passes the chief status to Arthur. When Taliesin traveled with the party to break the glyph on Mt. Claes, he stumbled upon the Dragon's Grave, where he meets a hatching dragon. He also learned about Arawn's past and his suffering there. Through those past visions, he learned how to play the Song of Creation, which he eventually used it to stop the blizzard to cause the entire mountain to crumble onto the Dark Army.
He returns later to assist eveyone in the final battle with an army of dragons volunteering their assistance to the party.
He is foretold that if he becomes Chief of the Brigantes he will offend the Heavens, and will become a victim to a beast. Much to the prophecy, Taliesin now cares for his beloved dragon that he met and is practically enslaved by it due to its constant hunger, which he doesn't mind.