Quentin Robinson has been a ship's doctor for twenty years, and has served on the Kestrel for five years. He is human, from Earth, and has no living family.
The crew refer to him as 'Quack' (a nickname used for those selling medicine of dubious quality in the days before modern medicine), but not to his face. The nickname isn't helped by his large nose and collar-length hair. Although an older man, he keeps himself fit, and can be quite sharp with any crewman who doesn't do the same. Most of the time though, he is quite gentle with his charges, as we see when he treats trainee Tabitha Enns for space sickness, and promises not to tell anyone.
He has his own secrets, which come out in Intruders, and directly lead to him having to leave the Kestrel. Firstly, he didn't pay much attention to his low gravity training, and had avoided needing it ever since. Likewise, he had managed not to need to wear a space suit. He left that sort of thing to his paramedics, and dealt with casualties once they got to sick bay.
He needs both when a casualty from a mining accident needs to be taken to the Kestrel via the surface of an asteroid. His inexperience is his undoing. His injury cannot be treated quickly enough for him to be available for Kestrel's special mission. He is replaced by Matthew Ky, who we will meet later.