"Now, I am the first to appreciate a joke," said Arthur and then had to wait for the others to stop laughing.
Deadpan Snarker: Clearest and strongest in the original radio drama, where his sarcasm actually makes him famous among the bird people of bird people of Brontitall. In other versions this side of him is usually downplayed quite a bit, but he still occasionally gets in a few sarcasms.
Ford: How would you react if I said I'm not from Guildford after all, but from a small planet somewhere in the vicinity of Betelgeuse? Arthur: I don't know. Why, do you think it's the sort of thing you're likely to say?
Genre Savvy: Arthur always suspects the worst will happen, fitting considering he's a character in a very cynical series.
Pajama Clad Hero: Despite his dressing gown being iconic, Douglas Adams himself didn't realise that Arthur would logically still be wearing it after leaving Earth until the TV series was made, meaning that Arthur's pajama clad status was not mentioned in the radio series or the first two books.
Unknown Rival: Agrajag, a reincarnating being who Arthur has unknowingly killed many, many times over. Agrajag is understandably annoyed.
"Hey, you sass that hoopy Ford Prefect? There's a frood who really knows where his towel is."
Radio: Geoffrey McGivern; TV: David Dixon; Film: Mos Def
Bearer of Bad News: When you tell everyone you meet that the world is going to end in a few minutes, you are indeed a bearer of bad news.
Cheshire Cat Grin: Smiles "a little too broadly, giving people the unnerving impression he was about to go for their neck."
Creepy Blue Eyes: In the TV series. The actor tried using purple contacts but they were just gilding the lily of his already creepily intense eyes. (In And Another Thing... it's revealed that Ford finds it relaxing to not blink and can go for eight minutes without doing so — he's timed it and wonders if it's a new record.)
Deadpan Snarker: Actual verbal irony is not a concept they have on his planet, but he still manages to be a smart-ass without it.
The Hedonist: All he wants from life is to drink copious amounts of alcohol and dance with girls.
Herald: Sends Arthur on an adventure, but far from a traditional example and in no way a mentor to the protagonist.
Heterosexual Life-Partners: With Arthur. Of all the humans to save from Earth's untimely destruction, Ford chooses his best mate in the hopes that they can bum around the galaxy together and have a laugh.
Human Alien: Despite presumably being the same species as Zaphod (they're semi-cousins) Ford doesn't have any extra appendages and looks perfectly human, give or take a disturbing smile.
Bizarre Alien Biology: Two heads and thee arms. Depending on the version of the story, they're either natural for his species of Zaphod added them himself for a multitude of reasons.
The Fool: Mostly because many of his apparently random impulses are actually related to memories he erased from his mind years ago. He'd rather not think about it too hard.
Genius Ditz: He's a hedonistic, self-absorbed thrill seeker but he has moments of brilliance. His past self was much smarter, given the master plan he cooked up to meet and confront the ruler of the universe.
The Load: In the movie, he spends the latter half of somewhere between this, The Millstone, and vaguely useful, because he's missing one of his heads. Ford actually has to drag him around in one or two scenes. Also, when they're getting shot at, he apparently thinks it's a dance party. Fortunately, Vogon soldiers make even the Imperial Stormtrooper Marksmanship Academy look good by comparison.
Multiple Head Case: Subverted in most adaptations (where there's no distinction between the two heads), pretty much played straight in the movie, and zigzagged in the sixth novel, where the second head has a distinct personality after being removed and attached to the Heart of Gold.
In the film, Zaphod's second head often sports a Slasher Smile while making serious threats, like promising to pull Arthur's spleen through his mouth.
Screwy Squirrel: He is a complete scoundrel, will almost screw anything over for personal gain, and far from a role model. Practically the reason why he is the president of the galaxy in the first place, actually.
That Man Is Dead: In the back story he lobotomised himself to keep his plans secret even from himself. However, turns out that the 'new him' hates the old one and actively works against those plans.
Tricia MacMillan/Trillian Astra
"It was either this or the dole queue again on Monday."
Promoted to Love Interest: In the film, she's Arthur's love interest. In every other version, Arthur tried unsuccessfully to flirt with her at a party before the events of the story and that was that.
Race Lift: Described as "vaguely Arabic" in the first book, implying she has South Asian ancestry. Played by white actresses in both the TV series and the film.
The Smurfette Principle: The only female character of note until Fenchurch and Random came along in the fourth and fifth books, respectively.
Women Are Wiser: She gives the impression of having her act together even when she doesn't. She's easily the most sensible and mature person aboard the Heart of Gold.
Marvin "the Paranoid Android"
"Life. Don't talk to be about life."
Radio: Stephen Moore; TV: voiced by Stephen Moore, body of David Learner; Film: voiced by Alan Rickman, body of Warwick Davis
Catch Phrase: "I think you ought to know I'm feeling very depressed", "Life? Don't talk to me about life." some of the first two lines he says when introduced in every continuity of H2H2. There's also his "Here I am, brain the size of a planet..." speeches.
The Chew Toy: He literally gets treated like crap (or makes himself believe he is, at times) by almost everything in the entire universe. The fact that he is several times older than the universe itself in the later books doesn't help much, either.
Flawed Prototype: Marvin was the unsuccessful prototype for the emotion chip. Everything else that has it is irrepressibly cheerful all the time— including Eddie, a ship AI who will cheerfully tell you you're about to be vaporized by nuclear missiles, and even the individual doors which all thank you for passing through them. Marvin hates them all.
Image Song: He had four such songs sung by Stephen Moore, his actor from the original radio and TV series.
Ridiculously Human Robot: Subverted. While humanoid, his emotion chip is supposed to emulate real emotions. Unfortunately, it does that too well, and only with depression.
Robot Buddy: Under certain definitions of the word "buddy". The marketing division of the Sirius Cybernetics Corporation was probably referring to a different robot when it advertised "Your Plastic Pal Who's Fun To Be With!"
At one point he is put in charge of all the computing for a robotic army and fleet that can take on the whole galaxy at once, and is still so under-challenged that he composes little poems to keep himself occupied.
Time Abyss: Oh so very, very much. In one instance he stays in one spot from approximately 1980 until the end of the universe. By the end of the series he is, by virtue of Time Travel, six times older than the universe itself! He is then brought back to life again because numerous characters that lived when he was created were still alive, and that went against the Lifetime Insurance Policy of Sirius Cybernetics Corporation.
"The first ten million years were the worst. And the second ten million, they were the worst, too. The third ten million I didn't enjoy at all. After that, I went into a bit of a decline."
"Hang the sense of it all and keep yourself busy."
Radio: Richard Vernon (first and second series), Richard Griffiths (fourth, fifth, and sixth); TV: Richard Vernon; Film: Bill Nighy
Expy: Of the Doctor in Life the Universe and Everything due to the book being an adaptation of a rejected Doctor Who serial by Douglas Adams, Doctor Who and the Krikkitmen. Both the Doctor and Slartibartfast are very old human-looking aliens who travel through space and time righting wrongs in a weird looking spaceship (a police box for the former, an Italian bistro for the latter). Ford and Arthur even act as companion stand-ins.
Noodle Implements: How Wowbagger became immortal involves a rubber band, a boxed lunch, and a high-powered particle accelerator. People who have attempted to recreate the circumstances that made him immortal have failed, looking very stupid (or very dead) in the process. Or both.
Who Wants to Live Forever?: Best shown by the fact that he has watched every movie ever made at least thirty thousand times. He's utterly bored, leading to his impossible goal of personally insulting every living creature in all of time and space.
Brick Joke: She is the subject of one, when the fourth book opens with a narration identical to that of the first book, then reveals that Fenchurch was the woman mentioned therein.
Distaff Counterpart: She's slightly awkward, very ordinary, extremely British and carries a great secret of the universe around in her head but she can't remember exactly what it is. Arthur without the y chromosome, really.