This posts throws up so many red flags I don't know where to start. The last thing you should be thinking about is the interest. The person giving you the money is getting an equity interest which is a security. Unless that person is actively participating in the business, you will have to comply with state and federal securities laws or you face civil and possibly criminal liability.
Once you get past the securities issues, the next concern is about your business entity and the entity agreement. Do you have those? If not, that is a huge problem. It sounds like you are doing both a lending agreement and equity. Those will have to be reflected in promissory notes, security agreements, owner agreements, and the like.
As to the interest on the debt portion, that depends on the business, the risk, the assets, so on. It is not uncommon to expect something along the lines of well over 10% depending on the risks.
Hire an attorney now or plan to be involved in lawsuits later.
This answer is for informational purposes only and is not legal advice regarding your question and does not establish an attorney-client relationship.
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