Jester Thomas is an adept pyromancer. He is available as a summonable character against Mytha the Baneful Queen. He will invade the player's world at the bottom of the Dragon Sanctum just before reaching the area with The Imperfect.
It is believed that Jester Thomas is in fact Magerold of Lanafir, as Magerold sells the Jester Set and is the only means of getting it, he is found just after beating Mytha the Baneful Queen and there is a line in the Jester Set that reads:
Jesters are more than festival fixtures;
some have a second face, hidden from
Which indicates that Jester Thomas has a hidden identity.