The character I have in mind..well, he's not the antagonist, that's for sure. He's sort of skeptical but I have someone much better for that, and he is most definitely not a sidekick. But because his journey is a distorted mirror of the MC journey, and because that journey is so central to the heart of both their characters, they have a big impact on each other. They have the sort of influence on each other which may seem smaller than other more immediate problems, like the problems the Contagonist creates, but over the long term this character's impact and alternative journey is enormous.
So, I guess that's the question...what's the downside of making the IC a Passenger, if there is one.