Hello Dean

Dean & Castiel in 8.07 A Little Slice of Kevin.

Hello, Dean. Cas, we've talked about this. Personal space? – Castiel & Dean, 5.03 Free to Be You and Me

'Hello, Dean' is a greeting said by Castiel.

In an interview with Entertainment Weekly, Misha was asked what Castiel's catch phrase was and he answered 'Hello, Dean'. Here is an account of every time Castiel has said this phrase throughout the show.

4.03 In the Beginning

Castiel: Hello, Dean. What were you dreaming about?
Dean: What, do you get your freak on by watching other people sleep? What do you want?

4.22 Lucifer Rising

Dean finds himself in a large, lavishly appointed room. The walls are painted white with gold trim and accents, and a marble table stands in the center. Castiel is there.
Castiel: Hello, Dean. It's almost time.

5.03 Free to Be You and Me

Castiel: Hello, Dean.
Dean: Cas, we've talked about this. Personal space?

6.06 You Can't Handle the Truth

Dean: Castiel? Hello? Possible loose nuke down here, angelic weapon. Kinda your department. You hear that, Cas?
Castiel: Hello, Dean.

6.17 My Heart Will Go On

Dean: Cas!
Castiel: Hello, Dean. Sam.

6.20 The Man Who Would Be King

Castiel: Hello, Dean. Are you all right?
'Dean: Yeah, I'm...I'm fine. How are you?
Later in the episode...
Castiel: Hello, Dean.
Dean: How'd you get in here?

6.21 Let It Bleed

Castiel: Hello, Dean.
Dean: You're like a brother to me. So if I'm asking you not to do something, you gotta trust me, man.

7.21 Reading Is Fundamental

Castiel: Hello, Dean. Sam.
Sam: Hey, Castiel.

8.07 A Little Slice of Kevin

Castiel appears in the mirror in the bathroom. Dean straightens up, sees Castiel in the mirror and turns around.
Castiel: Hello, Dean.

11.11 Into the Mystic]]

Lucifer/Castiel: Hello, Dean.
Dean: Right, yeah. We don't hear from you for days, you show up, you start wrecking the joint.

11.14 The Vessel

Lucifer answers the phone, and grimaces as he imitates Castiel's voice.
Lucifer/Castiel: Hello Dean.

12.07 Rock Never Dies

Dean: Hey, Cas.
Castiel: Hello, Dean.
Dean: You still living out an '80s buddy comedy with Crowley?

13.07 War of the Worlds

Asmodeus(impersonating Cas): Hello, Dean.