It would be a lime (which would make it R rated) if you went into certain details about the gardevoir or the other character. Since you said there won't be anything explicit, I believe your story would be between PG-13 and 15. If there's less violence then it would lean towards PG-13, but more violence leans towards PG-15, or even R if you do it with gore and such. Hope that helps in judging the rating.
As I adventured into math and ideas, that old desire for fan fiction came back.
This review took at least an hour to write, by the way.