Trillian is one of Arthur's friend from earth and they met at a party on earth and now that they are a few of the only people alive is very surprising so Trillian is going to help them by taking them to Magrathea in the Heart of gold. Arthur comes aboard the heart of gold and the ships android brings them to Zaphod and Trillian who were on the main deck of the Heart of gold
Trillian left from earth who left six months before the planet was destroyed by the Vogons. Trillian has a degree in maths and another in astrophysics and that is why she decided to travel to outer space, and one night on earth while attending a party she met Zaphod who was paying earth a rare visit. On that same night Arthur tried to flirt with her but Zaphod came in and ruined it.
Trillian is a very smart person who left earth 6 months before it was obliterated by the Vogons, and in the time that she was still on earth she met someone known as Arthur dent and now they find each other again on the Heart of gold in the middle of space sailing towards the planet of Magrathea.