Is Mr. Robot in Elliot's head?

Mr. Robot is all in Elliot's head. He is, in fact, Elliot's mental construct of a version of his father who had died years ago. Darlene confirms in eps1.8 (episode 9), that Elliot is the one who founded FSociety.

So, in the show, Mr. Robot, we're falling down the rabbit hole that is Elliot's mind. As the unreliable narrator of the show, we experience what Elliot experiences and we follow along with him.

In an article from the Observer:

You knew all along, didn’t you?” Elliott Alderson asked us, his collective imaginary friend, in last night’s episode of Mr. Robot. And the answer, at least on my part, was: “did we?”

I mean, of course it was there all along — Christian Slater’s Mr. Robot is simply Elliot’s own Tyler Durden, a manifestation of his guilt-ridden and damaged mind molded in the image of his father. Of course Mr. Robot isn’t actually Elliot’s long-dead-or-so-we-thought dad, who had somehow been hiding his existence from his own son for 20 years. It was all there, the way Elliott would sink into the background any time Mr. Robot addressed FSociety, the way Mr. Robot would participate in discussions but the other members wouldn’t exactly acknowledge his input. Mr. Robot was always in the backseat of the car, or appearing at just the right moment not to interfere, but to dispense advice. We knew. We knew all along.

As an added note, the piano piece playing at the end of this episode of Mr. Robot is a piano cover of The Pixies' "Where is My Mind", which is the same song playing at the end of Fight Club.

Fight Club version (The Pixies):

Mr. Robot Version (Maxence Cyrin):

After watching s04e07, it occurred to me that there has been a lot of times when certain characters see Mr.Robot while others do not (that's obvious). However, did you notice when the thunder and lightning get really intense? It's always triggered by Elliot's anger/despair.

In the end of the episode Fernando says,

You are the storm and it's the rest of the world that needs to run for cover.

Spoiler:

Then When Krista stabs Fernando the lights go out. While the entire episode was shot like a stage play I think there's more to it than lighting and effects. This whole show has been all in Elliot's head, as each character dies so does a piece of Elliot's DID fade away.