So I'm on the verge of trying to join the military (probably Air Force or Navy) but I'm concerned about my background check. I have an ex-girlfriend who is ideologically opposed to any and all things military and would not hesitate (she has said this) to lie, cheat or steal to keep me out of the service, even now that we are long since broken up.

She and I leased an apartment together, so there's a chance a background investigator will talk to her and I'm worried about what she might say.

Is it guaranteed they will talk to her?

I can not see them contacting them at all.