I’m investing a company 18%. Can the larger shareholder fire me on the spot? When should I get my investment back?

Your investment in a corporation (you mention the largest shareholder, so I'm assuming the "company" is a corporation) is separate from your employment by that corporation.  Absent a contract which limits the reasons for which you can be fired, you can be terminated from your employment for almost any reason.  Termination of employment, however, would not divest your 18% ownership stake in the corporation (again unless there is a contract providing otherwise), and you would still have the shareholder rights you had before you were fired, for example to a pro rata share of dividends.  Nor are you entitled to get your investment refunded because you are no longer employed by the company (again unless there is a contract providing otherwise).  An investment is not a loan; there is no guarantee that you will ever get it back, you just hope the corporation does well so that the value of your stock goes up, and so you can receive dividends.