Mr. Adams

Mr. Adams is a man.  He appears in Letters I and III.  He loses a brother-in-law in Letter I and talks about God and morality in Letter III.

Mr. Polk

The only information about Mr. Polk is that he "looks about the same age as Mr. Adams."

Mr. Buchanan

He is definitely a male in his seventies or perhaps eighties.  He is of Italian origin and has grandchildren.  Well, I suppose that should be past tense now.  In Letter IV, he lectures about universities in America.

Mr. Taft

Blond-haired, blue-eyed, and twenty-three years old, Mr. Taft is described as Ganymede.  In Letter V, he introduces the letter writer to Dr. Morbus, and later describes how the Party fashions the logic of the guilty majority and the sin of firstness.  May still be in training, per the letter writer.

He makes an appearance again in Letter XV, where he discusses the so-called "Party-approved reparative political attitude."

Mr. Cleveland

Mr. Cleveland is not described.  All we learn is that he speaks Greek.  In Letter VI, he discusses how the Party encourages Americans to place passion for reason.  He also talks about the social phenomenon of guilty until proven innocent.  In Letter XIX, he takes the letter writer to the high school to learn the First Principle.

Mr. Grant

He is not described physically.  In Letter VII, he introduces the letter writer to Professor Babel, who lectures on his theories to restrict the use of language and takes them to visit the bonobos.  In Letter VIII, Mr. Grant accompanies the letter writer to meet with Professor Eris, who is working to have Americans place greater value on opinions over facts.  She (Professor Eris is described as a young Asian woman) also discusses outrage and banned words.  There is also mention of Mr. Grant's Theory, which states that segments of the electorate can be swayed when they are given the possibility to own property.  Mr. Grant knows a great deal about medieval literature.

Mr. Harding

Mr. Harding is unusually well informed about Africa.  No physical description of him (or her) is given.  He works to break down affective bonds between people, especially by promoting hate on social media, as described in Letter IX.

Mr. Monroe

Mr. Monroe is not described.  He is a gastronome.  In Letter X, he talks about food, set-class theory, and perpetual war among the proletariat.

Mr. Hoover

No description is given of Mr. Hoover.  He has an interest in viticulture.  In Letter XI, he talks about how the Party manipulates a phony right-left divide on various issues.  He also shows the letter writer the Emissary, which is kept on public display somewhere.

Mr. Fillmore

After having shown the letter writer through one of the Party's more amorous facilities, Mr. Fillmore (who is not described) talks about varied subjects in Letter XII, including reality, the structure of the Party, the social pecking order, etc.

Mr. Garfield

Although no description of Mr. Garfield is provided, I'm inclined to believe he is male, based on the particularities of his pilot.  However, that is not supported by the text.  In Letter XIII, he takes the letter writer to an Indian Reservation to meet with John.

Mr. Van Buren

Mr. Van Buren was in the newspaper business for over fifty years, which leads me to think he is most likely male, although that is not supported by a textual description.  He is an expert on the baroque.  In Letter XIV, he talks about the current state of the media in America and then takes the letter writer to see Dr. Morbus.

Mr. Washington

In Letter XV, Mr. Washington talks to the letter writer about the then-President of the United States.  In Letter XVI, he goes off on a wide range of issues, including Congress and the Supreme Court, giving examples from recent history.  He is a man,

Mr. Lincoln

Mr. Lincoln "looks like a New Englander," which isn't much of a description.  He is interested in the writer's country.  In Letter XVIII, he mostly recapitulates what other Presidents have already brought up, though be does go on about how great organized labor used to be.

Mr. Jefferson

He gives a personal summation about the Party's meaning in Letter XIX.  Mr. Jefferson is male.