Mr. Grey, also known as Thompson and #2222, is a government worker who has been assigned to the Farmer family in the Witness Re-Establishment Program.
We are first introduced to him as "[t]he gray man" (17.1). In an interview with Brint, Adam repeats these words three times in a row and then quickly retreats into himself. Over the next two sessions Adam doesn't say a word. We're pretty sure this is a sign that the gray man important, wouldn't you say?
Once he finally opens up, Adam explains why he calls him the gray man (and confirms that his name actually was Mr. Grey):
"[T]here was something gray about him. His hair was gray. But more than that: to me, gray is a nothing color and that's how Mr. Grey seemed to me. Like nothing." (22.1)
He's nothing to the reader, either, until halfway through the book. We aren't introduced to him until Chapter 17 (and even then, only with his name).
We don't get much of a description of Mr. Grey. Like Brint, he's a mysterious character. Also like Brint, he's supposed to be helping the Farmers, but there's something fishy about the situation. He's always asking Adam's dad for information about the scandal that caused their relocation. Adam himself notes the similarities between the two men (25.14).
Mr. Grey has complete power over the Farmer family – Adam's mom goes so far as to compare him to God (26.15). Yet for all the threat, Adam also notes that Mr. Grey would have been a good target for one of Amy's pranks (22.9), suggesting that he's kind of a doofus. This shows that someone's position can give him incredible power or, for that matter, take it away. Mr. Grey, who otherwise might be a target of a Number, has complete power over the Farmers' lives. Adam's dad, on the other hand, who is smart as a whip and funny to boot, has to live according to someone else's rules.