You have to understand the meaning of "sponsorship" for a Schengen visa.
- You will not be believed if you try to promise that she will leave the Schengen area again. Once she is in, you cannot make her leave.
- You will not be believed if you try to promise that she is a law-abiding citizen who would not overstay. Once she is in, you cannot make her leave.
What "sponsorship" means is an explanation of her source of funds. Nothing more, nothing less.
- Some people borrow money to immigrate into the EU, with the plan to overstay, work, and repay the debt. You must explain why you would give her the money as a gift, with no expectation of repayment. That means explaining your relationship.
- Some people spend all their money to get themselves or a relative into the EU, with the plan to overstay, work, and send money home. You must explain why you would spend the money on a luxury like tourism. That means explaining your income and expenditure, and also hers. Legal, documented income in excess of reasonable living expenses are more important than lots of money simply sitting in your account.
Only say that she is working for you if that is the truth. Everything has to fit together. If she is working in your company, does she have a payslip, a bank account with regular deposits, the usual taxes? How does that fit with the completion of her studies?
Lies will only make everything more complicated. What you need is a simple, reasonable explanation. You are a couple. You earn enough to afford the trip for both of you, and the relationship explains why you share your money. It is perfectly normal that not every couple is married. It is perfectly normal that one partner earns more than the other.