Dickon is the younger brother of Samwell Tarly. He became their father Randyll Tarly's heir after he disinherited Samwell by forcing him to join the Night's Watch.
Dickon is athletic, a skilled hunter and excellent swordsman, though not particularly bright and also quite arrogant and condescending. Naturally gifted at martial endeavors, he is his father's favorite child. After Randyll Tarly banishes his old brother Samwell Tarly to the Wall, he designates Dickon as the heir to Horn Hill and the Valyrian steel sword Heartsbane, which has been in the family for five hundred years.
When Samwell Tarly returns to Horn Hill with his "lover" Gilly and "their child" Little Sam, Dickon and his father Randyll are out hunting. Later at dinner, he is cordial but cool towards his older brother and Gilly. When Sam asks him whether they're eating the deer that he and Randyll hunted, Dickon replies that what they're eating was in fact last week's haul and that they did not have time to cure today's catch. He also engages in dinner table conversation with Sam about hunting beyond the Wall. Sam tells him that there are no deer and elk to hunt but only rabbits.
After Gilly tells the hosts that Samwell killed a White Walker, Dickon expresses amused disbelief and claims that White Walkers are just a myth. He is present when Randyll tells Sam that he will find work for Gilly as a servant and foster Little Sam as his grandson, but that Samwell is no longer welcome at Horn Hill. Undaunted, Sam takes his "family" away and also steals the family heirloom Heartsbane.
Second son to Randyll Tarly, Dickon has interests that are more in line with his father's. This trait endeared him to Randyll, who forced Sam to renounce his birthright so that Dickon would inherit Horn Hill.