Previous thread: http://boards.straightdope.com/sdmb/showthread.php?t=669054
The letter this time is
H.
IQ1: Are you the mother of Ishmael?
IQ2: Did your description of a hippopotamus reveal the fact that you had never seen one?
IQ3: Were you famously portrayed by Dan Blocker?
IQ1: Are you known as Vulcan in Rome?
IQ2: Are you a spicy ginger pop star from the 1990s?
IQ3: Are you an iconoclastic author, poet, and punk rock icon?
Not Hagar, Hippocrates or Hoss.
Not Haphaestus or Geri Halliwell; dunno the third. Take a DQ.
Henry Rollins
DQ: Are you real?
H.
- Real
IQ: Did you write the 1st written code of law?
I don’t know about Hippocrates, I was after someone else. Disnae matter…
IQ1: Did you play The Doctor?
IQ2: Are you known as The Undercover Economist?
IQ3: Do you have a falcon head?
I’m not Hammurabi.
I’m not Christopher Heddleston, dunno the second, or Horus.
IQ: Were you the last prisoner at Spandau Prison?
IQ1: Are you an English TV character played by an American actor with the same initials as the character?
IQ2: Did you create the talented Tom Ripley?
IQ3: Are you the proposer of a possibly-detected particle?
Horus is correct.
Christopher Eccleston is the Ninth Doctor in the 49 year old series ‘Doctor Who’, but he’s not an ‘H’. William Hartnell, the First Doctor, was who I was after.
‘The Undercover Economist’ is the title of Tim Harford’s first book, and also the title of his blog/column with the Financial Times.
Now - DQ1: Are you male?
DQ2: Are you living?
Not Rudolph Hess.
Dunno the first.
Not Patricia Highsmith.
Not Dr. (Prof.?) Higgs.
H.
- Real
- Male
- Deceased
Correct re Patricia Highsmith and Peter Higgs of boson fame.
Jonathan Higgins, played by John Hillerman, was Tom Selleck’s sidekick in Magnum P.I.
My DQ: Were you alive at any time in the 20th century?
IQ: Did you go to jail because of Watergate?
Not H.R. Haldeman.
H.
- Real
- Male
- Deceased
- Alive at some time in the 20th century
IQ: Were you an aide to FDR at Yalta?
Not the cadaverous but irreplaceable Harry Hopkins.
EH-No idea what that means, but it made me LOL! 
IQ1: Did you and your colleagues take over Nakatomi Plaza in an unsuccessful heist?
IQ2: Are you also known as “Papa Bear?”
IQ3: Did you turn pro when the going got weird?