Symbolism, Imagery, Allegory
Yes, poor little Faith is way more of a symbol than a character. And what does she symbolize? Well, uh, faith. Obviously. Just check out the way Goodman Brown talks about her:
• "Is that any reason why I should quit my dear Faith and go after her?"
• "And what calm sleep would be his that very night, which was to have been spent so wickedly, but so purely and sweetly now, in the arms of Faith!"
In other words, "Faith" is a stand-in for the precepts and beliefs of his Christian faith. All the evil people he meets in the woods are trying to lead him away from her/it, but he resists. Still, the awful things he sees means that, when he eventually returns, he finds her/it not quite as comforting as it used to be.