I'd like to avoid the accounting/filing headaches and costs that come along with legal partnerships, but would still like to retain this investor.
If your business is currently structured as an LLC, you can admit a 10% equity holder as a limited member/partner, which means he holds an equity stake in the company, but doesn't participate in the day-to-day management activities of the business - that responsibility remains with you. You'll have to amend the capital schedule in the LLC operating agreement to admit the new partner. If the LLC was previously only owned by you, the admission of a second owner means the LLC is now a partnership for U.S. tax purposes, so you're required to file an annual Form 1065 partnership tax return. If you don't want to create a partnership, you could explore having him contribute the funds in the form of a debt arrangement, rather than an equity interest.
