James is the traditional antagonist at the beginning of the Autobiography. Ben Franklin doesn't even really want to apprentice with his brother, but is strong-armed into it by his father. James keeps Franklin from doing the kind of work he wants to do, doesn't treat him with respect, and beats him sometimes. He even expects Franklin to "front" publication of his newspaper after doing some jail time. This kind of mental and physical abuse of a youthful protagonist is typical antagonist behavior. Even though they're family, we get the sense that there's no nepotism here. If it weren't for the legal fluke that gave Franklin a way to sneak out of the apprentice agreement, he probably would've been stuck working for James till he turned 21.
This snake-in-the-grass starts out by being sweet to Franklin, encouraging him, and offering him support in becoming a small business owner. His early encouragement is so emphatic it's no surprise Franklin takes him seriously. He doesn't mean a word he says, though, and Franklin ends up stranded, penniless, in England because he listened to those words. It's not clear whether Keith is deliberately malicious, utterly scatterbrained, or just two-faced, and the text doesn't provide us with motivation for why he behaves in this way. Even if his actions aren't deliberate, they have the potential to be just as damaging. In classic turncoat behavior, this friend becomes an enemy, deserting our protagonist at one of his most vulnerable moments.