Little is know about Cletus life before the events of the game, he is portrayed as a friendly, stereotypical hillbilly living in Sandy Shores. He appears to be a military veteran, as he can be seen wearing dog-tags around his neck.
Events of GTA V
Trevor meets Cletus for the first time in Sandy Shores, Cletus will be hunting some coyotes and will enlists Trevor to do some hunting challenges to see if Trevor is a good hunter. The challenges consists of shotting satellite dishes, tires of moving vehicles and coyotes. After Trevor complete the challenges, Cletus asks Trevor to meet him in Bayview Lodge in Paleto Forest, for some hunting lessons.
In his second meeting, Cletus introduce Trevor to some tips to go hunting, like using the wind to not be detected by the deers or using a deer call to locate animals. After Trevor kill all the necessary deers, Cletus will call Trevor to make a proposal: if Trevor kill any animal that he finds and send to Cletus a photo of the corpse of the animal, Cletus will sell the animal meat and share the profits with Trevor. After this, the Hunting activities will be unlocked.
Cletus then continue to make appearances in the game only in the hunting minigame, he will send text messages to Trevor with comments about the corpses of the animals that Trevor kills and how much he thinks they are worth. He will also send challenges to Trevor complete along the hunting session, that include killing animals in a certain way or taking pictures of the corpse of specific animals. Completing this challenges help the player reach the gold medal in the hunting sessions.