A side agreement as a valid document to protect the shares of a foreign owner of an LLC

Is a side agreement considered a valid enough document to protect my company?

I own a car rental company and, by law, I was told it has to be an LLC, so I brought a partner who is really a nice guy.

I want to protect my business as I am the full owner and all the assets are mine; including the luxury cars, etc.

I only give the sponsor a fee yearly. I’m worried if something ever happened to my sponsor, God forbids, what will happen?

Can any of his family try to take his share even though the 100% company is funded and run by me but his share is 51% on papers only?

Is the side agreement valid?

Is there anything I can do to protect my whole 100%?

Are there any documents I can make or use?