Hitchhiker's Guide 1

Arthur Dent is an average human, so when he is faced with the reality of aliens and advanced technology combined with the fact that his planet was destroyed to make a highway, he is just a little bit in shock. He has no frame of reference for anything in the rest of the Galaxy, so he spends a large chunk of his time being completely dumbfounded. All he really wants is to do is to relax and have some tea, something that he just can't seem to do. 

Ford Prefect is a friend of Arthur's from Earth. He is actually not an Earthling, but a Betelgusean who has come to Earth to write entries for The Guide. In his many years of research, he has expanded Earth's entry from "Harmless" to "Mostly harmless". He has traveled far in his life and understands most everything about the Galaxy that the average individual from a space-faring civilization does, so he spends considerably less time being gobsmacked than Arthur. 

Zaphod Beeblebrox, Ford's cousin and new galactic president, is an arrogant, fun loving person whose actions are about as illogical as the 113th congress. It is unknown whether he is actually extremely unintelligent, or simply acts like an ignoramus for attention or to escape from reality. 

Trillian is an old aquaintance of Arthur and that's it. She is quite possibly the least important and least memorable character in the book. She is introduced and left to be a backdrop for the rest of Hitchhiker's Guide. 

Marvin the Paranoid Android is a robot with the brain the size of a planet. In a matter of seconds, he can solve any and every problem in the universe many times over, except his own. He would be a lot more useful if he weren't so depressed and cynical. His overall unhelpfulness stems from his creators giving him a "Real People Personalities" personality, causing him to be thoroughly depressed and thoroughly depressing.