First - your co-founder needs to be someone you TRUST. Because it sounds like you are going to be handing off money management and expense management to them. And that requires lots of trust.
Second - they have to believe in YOUR vision. Doesn't mean they have to agree with every detail, but if you are handing off "marketing" then in many ways you are also handing off the specification of the features and functionality of the product. After all, HOW it works and WHAT It does are large parts of marketing. So you need someone who buys YOUR vision and not theirs
Thirdly - you need someone with a track record. Because otherwise that trust issue is going to be that much harder to deal with when you are in the hole up to your waist in mud and digging furiously.