The Question of the Titles of Wilhelm II
In 1802, when Napoleon Bonaparte threatened to make himself Holy Roman Emperor, Francis II decided to dissolve the office to thwart this move and so it was that the Holy Roman Emperor Francis II then became Emperor Francis I of Austria (Kaiser Franz I. von Oesterreich). It was the Holy Roman Emperor who raised Prussia to the status of a kingdom and when Otto von Bismarck was carrying out plot after plot to re-unite the German peoples, this time under Prussian leadership, he had quite a difficult time persuading Wilhelm II's grandfather, Wilhelm I, King of Prussia, to accept the title of "Kaiser". The first Wilhelm was, of course, a Protestant, and a very devout one, but he was nevertheless very uncomfortable and reluctant to take this title, on the quite obvious ground that there already was a Kaiser and that was the Kaiser of Austria, successor of the past kaisers of the Heiligen Roemisch Reich dem Deutschen Volkes which had been ruled by the Hapsburgs for centuries. Bismarck ultimately wore him down, but it was made a specific point that the Prussian king would be Deutscher Kaiser and not Kaiser von Deutschland.
The new German Empire was, in many ways, simply a new Protestant and more nationalistic version of the old Empire. They even adopted the name of "The Second Reich" to refer to the new empire, claiming then to be the successor of the Holy Roman Empire, which was the First Reich. Much of the culture of the German Empire was taken from the chivalric Catholic Empire of the Middle Ages. Wilhelm II's father even considered reigning as "Kaiser Friedrich IV" according to the number Holy Roman Emperors, but ultimately decided to confine himself to counting the kings of Prussia only. There were also similarities in the form of the new empire, especially in that it was a collection of semi-autonomous states rather than a single country, although the states certainly had less independence than they would have had in the First Reich. Each state still had its own royal, princely, ducal etc. ruling family, its own government, flag, uniforms, military organization and other trappings of sovereign nations. It was a federation of German states in which the presidency was held by the Kings of Prussia. It was really improper for the Kings of Prussia, or anyone else for that matter, to assume the title of Kaiser, when this was a title given by the Pope and so it was particularly odd for a Protestant king to assume this title.
Incidentally, at the end of World War I, Kaiser Wilhelm II, who was under extreme pressure to abdicate, considered for a time to abdicate the office of German Kaiser, but retain his legitimate title as King of Prussia, at which time the rest of the states could do as they please in regard to the wider German Reich. Ultimately of course, this did not happen and Wilhelm II abdicated completely as both King and Kaiser, though there were some rumors of perhaps the Wittlesbachs of Bavaria assuming the imperial throne, which, while certainly innovative in so far as the German Empire was concerned, was not entirely without precedent as the Wittlesbachs had in the past served as Holy Roman Emperors as well.