If the investor injected "any" capital into the rental business, then that investor "is" an owner. Just not in the traditional sense. From what I can fathom, this is not being operated like an LLC or a partnership. It sounds to me more like an S-Corp where your investor owns shares in that business.
you *REALLY* need to seek the advice of a tax professional in your local jurisdiction. Taking advice from this public user-to-user form for your specific situation could come back and bite your wallet BIG TIME in the future, if/when the IRS audits this. If things look questionable, and it *will* look questionable if you file this as a single member LLC or partnership, the IRS will come a knocking about 18-24 months down the road.
