SPOILERS AHEAD FOR THE ENTIRE SERIES.
READ AT YOUR OWN RISK.
Near the end of Tales from the Borderlands episode 5, The Vault of the Traveler, the player must pick three teammates to join the fight against the vault guardian. Only select characters will be available to pick from depending on the choices you’ve made throughout the rest of the game. Here’s what you need to do throughout each episode in order to unlock each character as a playable option.
Felix, the Inventor*
Right at the end of episode 1 you must warn Felix about the bomb in the suitcase. If you do not, he will die. If you do, he’ll be around for the finale and give you some additional funds before the end.
August, the Brawler
Near the beginning of episode 3 Vallory will ask Fiona who is responsible for the Vault Key deal going south. So long as you don’t blame August, he will show his appreciation at the end and be willing to help you out.