Stump the German

[Eva Elisabeth]

Brilliant, Roger, the gentleman is Charlemagne, Karl der Große, or Charles the Great (742 - 814).

He was King of the Franks, King of the Lombards, and finally (on Dec.25, 800) coronated as the first recognized Imperator Romanorum *("Emperor of the Romans") in Western Europe since the collapse of the Western Roman Empire three centuries earlier. He united most of Western Europe during the Middle Ages and laid the foundations for modern France and Germany.

[* by the full title "Karolus serenissimus Augustus a Deo coronatus magnus pacificus imperator Romanum gubernans imperium" ("Charles, most serene Augustus crowned by God, the great, peaceful emperor ruling the Roman empire")]

Charlemagne made Aachen, where I was born and raised, his imperial capital (and over the next 500 years, most kings of Germany destined to reign over the Holy Roman Empire were crowned in Aachen). The photo shows a mask reliquary, located at Cathedral Treasury in Aachen. In the cathedral is also the throne of Charlemagne and the subsequent German Kings:
   
Well I feel a bit sorry as for this hard trivia task now, but I was so thrilled to learn from the recently published book (hint #4) that Charlemagne and his adventurous life played, on the evening of April 14, a major role for two MAIN characters in the assassination saga - I couldn't resist...

Now - who were these two well-known persons, and how was Charlemagne "amongst" them?

Hint #6: One was male, one female.
Hint #7: The original source the new book refers to is a well-known diary.