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was a friend to Arthur Spiderwick. It's assumed he was simply a writer friend or correspondent to Arthur, although it's possible that he was a relative of Constance Spiderwick.

Apparently, he was a pilot at the Royal Air Force in the U.K., but he lived in USA. He was also interested in Arthur's studies, at least more than Arthur's wife.

The only place where Robert is mentioned is in a letter to Arthur, found in the chapter about Goblins in the Field Guide:

Dear Arthur,
Considering what you told me of your studies, I thought you'd enjoy hearing some of the stories the boys at the Royal Air Force tell about a creature that interferes with the plane's mechanics. They call the chap a gremlin.
One of the pilots told his writer friend, so I thought I might as well tell mine.
Give my love to Constance. I will write to you again when I am back in the states.
— Robert