German Emperor

The emperor (German: Kaiser) was the head of state of German Empire (Kaiserreich) from 1871, the unification of Germany, to 1918, when Wilhelm II was forced to abdicate during 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 a few Harry Turtledove timelines, the Kaiserreich 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
Much of this is deductive based on OTL knowledge. The novel is set around 2096, when the Empire is ruled by "the Kaiser," who is never given a name. There are posthumous references to the historic Wilhelm II and the mysterious Wilhelm IV, whose biography and chronology are never discussed. However, the presence of his statue in San Francisco, which Germany conquered in 1956, is a strong indicator that he was reigning during that conquest. That there was a Wilhelm IV implies that a Wilhelm III did reign at some point.

Had the House of Hohenzollern continued ruling into the mid 20th century, Crown Prince Wilhelm would have reigned as "Wilhelm III". His son, Prince Wilhelm of Prussia, would not have been killed during World War II, and presumably would have reigned as Wilhelm IV in the 1950s. Thus, the administrators feel comfortable with identifying these princes with the Kaisers in the novel.

Southern Victory
Under the reign of Kaiser 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
Friedrich III is not mentioned in Southern Victory, nor any known Turtledove work, but the Point of Divergence would not seem to have affected him, so his reign between Wilhelms I and II is presumed. The identity of Wilhelm II's successor is open to some debate; see here and here for the reasons supporting the moderators' decision to refer to him as Wilhelm III.

"Uncle Alf"
In 1929, Kaiser Wilhelm II was in the 41st year of his reign over the most powerful nation in Europe.