The thing that really makes me think that is his appearance at Allsafe. No way some random stranger gets in there to sit at Elliots desk.
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Maybe the twist is that Christian Slater IS real. lol IDFK
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I have only watched two episodes so I don't have a lot of scenes to go off of. Off the top of my head:

Ep 1.0 Slater knocks on the door to the secret meeting place and the guy only looks at Elliot when he opens the door. No other scene (that I recall) would naturally lead to Slater talking to anyone.

Ep 1.1 Slater verbally interacts with the crazy girl but he steps in front of Elliot to do it and she never changes her focus from Elliot's face, Slater then announces to no one in particular that he's going to go sit in the back.

These have the same feel as the transitions in Fight Club. And I like the idea that the rest of the club are Elliot's other personalities. Very nice.
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The way he pops in and out in general implies to me that he's not real.
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Plus he never gives a straight answer in who he is, or how "they" can find him
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How does the drug dealer girl react when they find Marlene randomly in Elliott's apartment? If i remember right, I was expecting more of a holy shit response.
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Slater is totally a voice in his head. No doubt in my mind.
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I think she said something like "Nice.....I'll leave you to it" or something, like she's real. But I may not recall right.
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I thought Wellick was a dissociative personality with the off the handle beating up a transient guy, and how he singled out Elliot. Then came the all-homo butt sex, and after that I knew he was a different character and not a personality of Elliot's.
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I'm calling it now, Christian Slater is the mental manifestation of Elliot's Father.

"You betrayed his trust" then pushes him off the bridge like his Father pushing him out the window. ( Then he apologized, and said his Dad probably didn't mean it either)

Then there's " I'll never leave you" comment in the motel when Elliot was going through withdrawals.

Elliot has always said the only person he could talk to was his Father.
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I cant figure out if he's real or not. Some times I think he's part of his imagination but then other times like at the drug house when the guy outside interacted with him makes me think he's real.

He also interacts with other members of the crew and then they are seen out on their own doing things by themselves.

EDIT: I take back the drug house thing because now that I think about it that shit never happened. He wakes up and was never shot.
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That's like Tyler Durden talking to his crew while Ed Norton watched
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Been a long time since I watched Fight Club but I don't remember Durden carrying on a conversation with anyone but Norton. He made speeches but there wasn't return dialog, not that I recall at least.
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The car scene, the Mall (selling soap), Helena Bonham Carter telling her to shut up after banging her.

But, yeah Slater is his Dad
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He also takes that beating from lou. The owner of the bar who's basement they were using.
The store clerk who's license he keeps. And there are others.
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