What patreon tier do you have to be to unlock the extra characters?
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You don't need to be a patron to unlock the characters; everyone gets the FULL story and ALL characters. The patron-sponsored characters are those that were created with a handful of patrons who donated at a higher level but they are gradually being integrated into the story. Players/readers do not need to pay anything to unlock them. Other than early access, I will never paywall anything with BBN.
Edit/PS: The patron-sponsored characters currently integrated into the game are Connor and Seth. A third will be introduced in the next update, and the final two to come in the following updates.