"I saw him, Mr. Fudge, I swear it was You-Know-Who, he grabbed a woman and Disapparated!"
-- Williamson, in the Atrium of the Ministry of Magic after Dumbledore's battle with Voldemort (OP36)
Williamson is an Auror, who wears scarlet robes and has long hair tied back in a ponytail.
He accompanied Cornelius Fudge when the Minister arrived at the Ministry of Magic after the Battle of the Department of Mysteries (OP36).
Williamson as a surname comes from the name William (the "son of William"), which is derived from Germanic "wil" (will, desire) and "helm" (helmet, protection) (Internet Surname Database).
He was probably the scarlet-robed, ponytailed Auror seen in Auror headquarters with his feet up, dictating a report to his quill (OP7).
From the Web
Pottermore feature: Guide to Wizarding Careers
Harry Potter Wiki: Williamson
Tags: defence defense employment enforcement laws red scarlet Wizarding laws