A co-founder is someone who starts a company with you and owns a portion of the company similar in size to your portion. The exact details of how you work it out, how you structure the company and how you involve lawyers are up to you.
Generally, I think you should avoid founding companies with people you think are going to rip you off. That's not to say that you shouldn't have proper paperwork.
I've got a friend who wants to start up a website. He's already got the domain, the idea, and needs admin and webdev help. But there's no business entity yet. He wants to include me, and have me share in the fruits of our labor, but sees no reason to put anything on paper. I'm guessing once an LLC is formed, only his name will be on it.
I trust my friend. But I've also seen money do strange things to people, and was wondering what the options are.
Generally, I think you should avoid founding companies with people you think are going to rip you off. That's not to say that you shouldn't have proper paperwork.