Harry has at last found love at Hogwarts. His close friend, Colin Creevey, says that Harry is rarely seen out of the company of one Hermione Granger, a stunningly pretty Muggle-born girl who, like Harry, is one of the top students in the school.
It seems to me that "out of the company of Hermione Granger" is fine. I'm not sure what 'one' is doing there. Any thought?
Good question. A synonymous phrase for one there would be a certain:
"... rarely seen out of the company of a certain "Hermione Granger".
You can understand the word one and the phrase a certain there as meaning "a particular": in the company of a particular person who goes by the name "X" or who can be identified as "X".
It's the sort of phrase you'd encounter in a newspaper gossip column:
His passions are poker, craps, and nights in the company of a certain redheaded showgirl at the MGM Grand.
The senator was seen in the company of one Luna Walker, a former burlesque dancer at the Trocadero.
or in an evidentiary proceeding where things are being stated "for the record".
This is a bit formal usage, but it’s pretty standard: it means that the named person will not be previously known to the reader.
I might write to you, for example, and say that I visited Hollywood and met Arnold Schwarzenegger, and also had a fascinating chat with one Aaron Smith whom I met on a park bench. In the first case I’m assuming you know of that person; in the second, this is the first you’ve heard of them.
In informal speech you’d say “a person by the name of” or “a guy called”.
