Adramalech

He's Chancellor of Hell, aiding Asmodeus from the Dark Lord's palace in
Nessus. Another one of his titles is Keeper of Records. He has a legion
of spies and informants among the pit fiends and elsewhere, and writes
down all the information he gains in the Infernal Records, which
mortals call the Book of Fire.

His spies have, however, been subverted by his rival Phongor, Hell's Inquisitor.

The Infernal Records include
the truenames of all Baatezu and the rest of Baator's bookkeeping:
numbers, acquisitions, the transformation of Larvae and lemures, ranks,
powers, known and suspected alliances among the hierarchy, copies of
diabolic contracts, and rosters of baatezu minions and allies on other
planes.

In the modern era, most of this information is
delegated to the Dark Eight, but Adramalech presumedly still compiles
what they pass on to him.

From Dragon #76:

"Adramalech appears as an aged, gray-bearded and balding man, with
cold, wet eyes that change hue from emerald green through black to
fiery orange depending upon his mood (enjoyment to fury,
respectively). He has normal human feet and hands, small crimson
barbed horns and a crimson forked tail. He wears loose, flowing robes
of green, purple, russet, or black, and beneath these is a humanoid
body that is crimson and scaly down the back, and yellow-green and
slimy down the front."

From Dragon #28: (paraphrased for clarity)

He is not physically strong for a diabolic noble, and he is rather
helpless in the face of disorganization, which is why he's never aimed
for a higher status.