Grady is a character we hear about in No.5 as Charles Grady, and meet a Delbert Grady in No.53.
Because he’s played as British (the actor who plays him is British, but nothing is established by the role as him being British in the novel or the film), it introduces him into the American History narrative.
Yes, the film is being filmed in England, but it’s.. a film.. set in Colorado. His insertion as British changes things about the character.
His demeanor shifts drastically from when we meet him and to how the scene ends. In the beginning, Jack is the important one- literal words spoken- and as it ends, Grady is fully in control. It is because of this, and the type of information Grady provides Jack once he’s fully trusted that Grady is Jack’s “Tony”; an internal mechanism for the survival of Jack.
By No.69 happens, Grady is the authority over Jack, using peer pressure of invisible “others” to get Jack to promise to kill his family.
Whether Grady lets Jack out or Jack gets himself out through the guise as Grady is unclear.
In this image · No.53
Compare: Grady’s Change in Manner
Grady begins as flustered and deferential, apologizing over the spilled drink. Compare the controlled, severe figure in Image 1149. The gradual transformation parallels Jack’s shift from social charm to violence.
References: No.53.