Mr. Gillmer is the prosecutor who pits his wits against Atticus Finch in court. During the Tom Robinson trial, Mr. Gillmer represents Mayella Ewell, the plaintiff, while Atticus represents Tom Robinson, the defendant.

The text tells us that Mr. Gillmer is a balding, clean-shaven man between forty and sixty years old. During the trial, Mr. Gillmer shows apparent embarrassment at Mr. Ewell's racist speech.

“Why, I run for Tate quick as I could. I knowed who it was, all right, lived down yonder in that nigger-nest, passed the house every day. Jedge, I’ve asked this county for fifteen years to clean out that nest down yonder, they’re dangerous to live around ‘sides devaluin’ my property—”

“Thank you, Mr. Ewell,” said Mr. Gilmer hurriedly.

However, despite his discomfort at Mr. Ewell's words, he is not affected by the shallowness of the latter's accusations. Later, Mr. Gillmer's comments about Atticus also demonstrate the camaraderie and the lack of animosity between the two lawyers.

Mr. Gilmer called attention to the hot day by wiping his head with his hand. “That’s all for the time being,” he said pleasantly, “but you stay there. I expect big bad Mr. Finch has some questions to ask you.”

“State will not prejudice the witness against counsel for the defense,” murmured Judge Taylor primly, “at least not at this time.”

Atticus got up grinning...

By calling Atticus "big bad Mr. Finch," Mr. Gilmer suggests a possible amiable relationship between the two of them.

Jem later tries to comfort Dill, who is upset at how Mr. Gilmer has been speaking to Tom Robinson.

“That’s just Mr. Gilmer’s way, Dill, he does ‘em all that way. You’ve never seen him get good’n down on one yet. Why, when—well, today Mr. Gilmer seemed to me like he wasn’t half trying. They do ’em all that way, most lawyers, I mean.”

Although this is in part Jem's attempt to comfort Dill, there may be some truth in this statement. So, in the story, Mr. Gilmer is a somewhat reluctant prosecutor: although he too may be prejudiced against Tom Robinson, there are some indications that he may simply be doing his best as a lawyer.