Trillian

Trillian is played by Sandra Dickinson.

She is a girl from Earth, with a degree in mathematics and another one in Astro Physics.

She met Zaphod at a party in Islington (he just dropped in on his way somewhere). Apparently Zaphod’s chat-up line was, "Is this guy (Arthur Dent) boring you, why don’t yer come and talk to me, I’m from another planet!"

As Trillian confessed, he was from another planet, and why should she stay around Islington.  She was, after all, unemployed and Zaphod had a lot to offer (well, an extra head and arm at least!).

So he and she stayed together. She is the member of the crew who is, at times, the only one sane.