Is it possible to be the inviter?

Is it possible to be the inviter of my girlfriend?, even though I only have a student visa?.
And the guarantor would be my stepfather but I will be the one responsible for her expenses

If you're on a student visa, you can't bring your girlfriend to Japan. You're not a guarantor and it would have to be someone who is a permanent resident that can be. Which country are you trying to invite them from?

For what purpose?

IF it is for tourism/short visit, then it should be fine, as long as your gf can proof her return tickets, itinerary, expenses, etc. ( requirements differ from country to country).

I assume that gf country require visa for short visit/tourist. So, triple check with Japanese Embassy in gf country.

Throw in this for reference, Japan visiting visa

https://www.mofa.go.jp/j_info/visit/visa/index.html

(for Philippines)
https://www.thepoortraveler.net/2018/03/japan-visa-requirements-with-g...

I'm planning to bring her here in japan

Again .. your question/reply is just too vague.

Bring your gf for what purpose? Please be more specific?

1. Bring gf to visit short term/temporary 15days 30 days? .. possible, read above link.

2. Bring gf to live with you long long time .. No, you can't. gf is not wife. No visa applicable.

3. Gf want to live long long time with you ..
then go get proper visa, working visa. which you are not applicable.

geez .. (�|‸ლ) facepalm ...