German Emperor

The emperor (German: Kaiser) was the head of state of German Empire from 1871, the unification of Germany, to 1918, when Wilhelm II abdicated after World War I. There were three emperors in that time: Wilhelm I, Friedrich III, and Wilhelm II, all members of the House of Hohenzollern. The emperor also doubled as the King of Prussia (and took his numerical title from its kings as well, hence Friedrich III), as well as the president of the federated monarchies (the kings of Bavaria, Württemberg, Saxony, the grand dukes of Baden, Mecklenburg, Hesse, as well as other principalities, duchies and of the free cities of Hamburg, Lübeck and Bremen).

In several Harry Turtledove timelines, the German Empire (Kaiserreich in German) continued past 1918, and was ruled by a succession of emperors.

Curious Notions
In one alternate visited by Crosstime Traffic, Germany won the equivalent of World War I, and the emperor eventually became the de facto ruler of the world.

Note
While a "Wilhelm IV" is identified as having reigned in the text, his biography is never discussed. That there was a Wilhelm IV requires that Wilhelm III did reign at some point.

In OTL, Crown Prince Wihelm would have reigned as "Wilhelm III". His son, Prince Wilhelm of Prussia, was killed during World War II, an event that did not take place in the Curious Notions timeline. Thus, the administrators feel comfortable with identifying the Crown Prince as Wilhelm III and his son as Wilhelm IV. In addition, while dates are sketchy, it is strongly implied that Wilhelm IV was reigning in 1956, which is consistent with this figure's age.

Southern Victory
Under the reign of Emperor Wilhelm II, Germany soundly defeated its enemies in the Great War and emerged as the dominant power in Europe. Wilhelm's death and the ascension of |his son helped trigger the Second Great War, but again, Germany emerged victorious, and Wilhelm III reigned over a victorious Germany and a more precarious peace.

Note
The identity of the younger Kaiser in Southern Victory is one of the most perplexing of all the apparent gaffes in Harry Turtledove's work. (See Inconsistencies in Turtledove's Work.) When Wilhelm II dies in The Victorious Opposition, it is announced his successor is Friedrich I of Germany and Friedrich Wilhelm V of Prussia. There are several problems with this. First, Wilhelm II's oldest son went in OTL by the name Wilhelm, not Friedrich, and intended to use Wilhelm III as his reign name. Second, the Kaisers of OTL never used different names-numbers for their German and Prussian titles. With this in mind, as there had already been three Prussian kings named Friedrich, including Wilhelm II's own father, it would be impossible for any successor of Wilhelm II to be called Friedrich I. The matter confuses even more in Return Engagement when there is one reference to "Kaiser Friedrich Wilhelm," mixing and matching his two previously given designations. Subsequently, he is only called "the Kaiser" without mention of any name throughout the three remaining volumes. With so few clues, the administrators of this wiki have decided that the names "Friedrich I" and "Friedrich Wilhelm" are errors on Turtledove's part, and have chosen to identify Wilhelm II's successor in Southern Victory as "Wilhelm III".

Other Emperors
In "Uncle Alf", where Germany won a brief war in 1914 (but not in the same way as in Curious Notions), Wilhelm II is reigning in 1929 and, given Turtledove's record for tropes, will presumably reign until dying of natural causes in 1941.