Tricia McMillan, also known as Trillian, left the Earth after she met Zaphod Beeblebrox at a party in Islington. Arthur Dent was in attendance at this party. She has a degree in math and another in astrophysics. She brought two white mice with her, Benjy and Frankie Mouse.
She was slim, darkish, humanoid, with long waves of black hair, a full mouth, an odd little nob of a nose and ridiculously brown eyes.