What to call "secret defends"?

Now I do not mean secret police nor do I mean vigilantes. I also do not related to shadow government. My question is what do you call those who work in the background unknown to both the government and public eye to defend people? Similar to the M.I.B. from, well, M.I.B. (Men in Black) movies. I was wondering because I am currently writing a story about such a group. Is it called an agency? Please let me know. Thank you.
Answers

John

Black ops. The movie is The League of Extraordinary Gentlemen.

Anonymous

I would call my fictional agency something that sounds utterly innocuous. Examples: Optimal. The Working Group. The Annex.

Cogito

'Defend' is a verb. The noun would be 'defender'. Call them anything you like.

Clayton

yeah, depending on its size and organization. It can be a group, an organization, an agency, or anything else you could think of really.

The Lord Humungus.

The Deep State is the current name for these people.