Short answers, pulled from the story.
Kaiser is derived from the Roman emperors' title of Caesar, which itself comes from the personal name of the Julii Caesares, a branch of the gens Julia to which Gaius Julius Caesar belonged. The title passed into German through centuries of Holy Roman imperial tradition, parallel to the Slavic word tsar.
The German Empire (1871-1918) had three kaisers, all from the Hohenzollern dynasty: Wilhelm I (1871-1888), Friedrich III who ruled for just 99 days in 1888, and Wilhelm II (1888-1918), during whose reign the monarchy ended near the close of World War I.
Bismarck chose Deutscher Kaiser, meaning "German Emperor", over Kaiser von Deutschland, meaning "Emperor of Germany", to avoid implying dominion over all German-speaking territories. "Emperor of Germany" would have offended the Austrian kaiser, since Austria, home to many Germans, was not part of the new empire.
The four kaisers of the Austrian Empire (1804-1918) were Franz I (1804-1835), Ferdinand I (1835-1848), Franz Joseph I (1848-1916), and Karl I (1916-1918), all members of the Habsburg dynasty.
Anti-German sentiment was at its peak during the First World War, and the word kaiser became closely associated with Wilhelm II, German Emperor. That association gave the title considerable negative connotations in English-speaking countries that have lingered long after the war's end.
Several common German words derive from kaiser, including Kaiserwetter (sunny weather with a cloudless sky, linked to Franz Joseph I's birthday on the 18th of August), Kaiserschmarrn (a lightly sweetened pancake), Kaisersemmel (Kaiser roll), Kaiserfleisch, and Kaisersuppe. The prefix generally signals the highest or best of its kind.