Generally speaking, a Preferred Share class with strong protective provisions (such as the right to appoint a board member) could afford you significant protections but if the terms are too onerous relative to the amount of money and stage of the Company, it might prevent the Company from raising further funding downstream, thus hurting your investment.

You could also put all of the money in on a convertible debenture, which has some advantages but doesn't necessarily protect you.

The best way to protect your investment is to invest in good people that you trust. It's simple and true. Happy to talk through the particulars of your situation.