VISAs

Do you mean to stay permanently or for a brief period of time? If you mean a long period of time then she will need a D visa, which is totally a hassle, whichever way you pin it.

D visas are issued to:
- students
- people with a work contract (you could potentially hire her in your company, but you would have to prove that you sought a Bulgarian to fill the position first)
- people who invest a large sum of money
- retirees with a pension
- freelancers

There is also a weird category of "household partner" which may include your girlfriend? You would have to write/call her embassy to ask about it.

https://www.immigration2bulgaria.com/bulgarian-visas/ tells you the reasons she can get a d-visa.

Honestly, if you are EU or already have a d-visa, the most simple way to get your girlfriend here is to marry her. Otherwise to enroll in school or find employment.