Drafting an agreement stating roles

Who cares? I mean the question literally. It's your agency, you're the boss. Who cares what you call who and what an agreement says. Take ownership. Based on how you describe this partner of yours, it's obvious that you don't consider him/her an equal.

Be careful, words can have specific legal meanings, and consequences. For example partnership has a specific meaning to the Internal Revenue Service in the USA.

A partnership is the relationship existing between two or more persons who join to carry on a trade or business. Each person contributes money, property, labor or skill, and expects to share in the profits and losses of the business.

A partnership must file an annual information return to report the income, deductions, gains, losses, etc., from its operations, but it does not pay income tax. Instead, it "passes through" any profits or losses to its partners. Each partner includes his or her share of the partnership's income or loss on his or her tax return.

Partners are not employees and should not be issued a Form W-2. The partnership must furnish copies of Schedule K-1 (Form 1065) to the partners by the date Form 1065 is required to be filed, including extensions.

If this is not what you intend, don't call yourselves partners.

DO get an agreement in writing! It doesn't have to be elaborate, but it is important, really for the protection of both parties. I recommend discussing the concept verbally first, explaining that it is for mutual protection and exploring what the needs of both parties are. Offer to write something up, then present it as a proposed document for discussion. If you both agree, sign and you're done, otherwise, discuss the concerns, i.e., where the document does not meet the needs of one or the other party, discuss possible adjustments and iterate as needed.

As for content, be sure to state the ownership of the pertinent things like the business (you are making it an LLC, S-corp or other legal entity, right? ), the name and the concept (if it is likely an intellectual property item which needs protection). Also be sure to state under what arrangement the friend is working, i.e, as a partner (yes, there are legal and accounting ramifications for that), an employee (typically must be paid), an independent contractor (rate can be stated in another document, or by verbal agreement though I recommend written).

It's also ipmportant to include basic termination conditions. It can be something as simple as "The working arrangement can be terminated by either party at any time with written notice with all intellectual properties developed to that time being owned by the business ..." (careful about compensation at this point).

It is generally recommended that you consult a lawyer for such agreements to be sure they cover all the bases and there are no major hidden "Gotcha's".