I wanted to know if an investor is investing in your idea and for some reason it doesn't make the profit that was expected,and the investment has failed,is the person behind the 'idea' /business plan 100% liable?

Or is he not completely responsible as it was the investors decision to invest?

The investment funds are separate from the asset managers themselves. The asset manager, like a Fidelity or Vanguard, are not allowed to invest their own capital in the funds they manage, nor carry the fund on their own balance sheet.

I recently invested in a networking business for $500. It seems to be working pretty well for me, and am able to work with one of the top leaders (he retired at age 29). He's helping me run the business successfully, and I'm hoping to do the same for others as well! Feel free to message me if you're interested and I definitely wouldn't mind helping you. :)

If they "invested" then money then they lose money

If they "loan" the money then the person they loaned it to still owes it

(When you start a business its very important you distinguish between people who give you the money to get you started (they have "loaned" you the money) and those who have actually invested in the business (by buying part of it))

Failure, or less-than-expected earnings, are the risk the investor takes. I can't imagine any situation in which the entrepreneur would guarantee all, or even some, of the investors' investment.

That would depend on the agreement between the two parties.

Each situation may be completely different.