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.
You're confusing partnerships and corporations.
If someone owns 10% of a corporation they are simply a shareholder.
Many people create corporations and refer to themselves and other founding shareholders as 'partners.' This may be the source of the confusion. A partnership (general or limited) is a different kind of business legal structure.
What you may also be thinking of is a directorship. The investor may or may not want to be a director. This would give them a seat at the table for Board of Directors meetings.
In certain states and provinces, directorships lead to some types of personal liabilities.
If they want to be a director, you should make sure they get advice on what they may be exposing themselves to.