(Forward-canon — pending. The Ohara backstory and the Government’s move against Robin resolve at Enies Lobby / CP9 and in the Ohara flashback, ~Ch. 391+. The @0397 Ohara-origin of the warning is on-page, but the present-day follow-through — the WG’s actual move against Robin in the present — remains forward; status held pending pending the scorer.)
Earliest known caller
(Unknown.)
Why this matters
Oda flags the protagonist crew’s archaeologist as a Government-level danger via the mouth of an Admiral one full saga before the Enies Lobby war makes that danger the central plot. The warning lands long before the audience (or Luffy) understands why the Government fears Robin — the standard “authority states the stakes, the flashback explains them later” technique. Conservatively: Ohara is NOT named, the name “Kuzan” is NOT spoken, and Robin’s specific crime is NOT given on-page @0321 — all forward-canon.