Tricia Marie McMillan, also known as Trillian Astra, is a fictional character from Douglas Adams' series The Hitchhiker's Guide to the Galaxy.

Physically, she is described as "a slim, darkish humanoid, with long waves of black hair, a full mouth, an odd little knob of a nose and ridiculously brown eyes," looking "vaguely Arabic.

The trilogy later reveals that Trillian eventually left the party with Zaphod Beeblebrox, who, according to the Quintessential Phase, is directly responsible for her nickname.

In the books, which the third, fourth and fifth series follow, she saves the universe from the Krikketeers and later becomes a Sub-Etha Radio reporter under the name Trillian Astra.

This idea was scrapped after the "making of" book was written, and the scene revealing Trillian's heritage (by the mice, to Arthur, on the Earth Mark II) was re-written.

In The Hitchhiker's Guide to the Galaxy novel, she is described as follows: "She was slim, darkish, humanoid, with long waves of black hair, a full mouth, an odd little knob of a nose and ridiculously brown eyes.