Fritz originated as a German nickname for Friedrich, or Frederick. The following characters in the works of Harry Turtledove are known as only Fritz:

Fritz (Worldwar), a German soldier and minor character in Worldwar: Striking the Balance.
Fritz (End of the World), a "throwback" in "It's the End of the World As We Know It, And We Feel Fine".
Fritzi (Iron Heart), a German veteran and minor character in The Man With the Iron Heart.

The following characters have Fritz as a first name:

Fritz Bittenfeld, fictional tank crewman in The War That Came Early.
Fritz-Julius Lemp, historical German U-boat commander and POV in The War That Came Early series.
Fritz Gustafson, fictional US Navy Petty Officer in Settling Accounts.
Fritz Pfeffer, historical dentist referenced in "The Eighth-Grade History Class Visits the Hebrew Home for the Aging".

"Fritz" was also a nickname for Germans used by their opponents during both the First and Second World Wars. Turtledove has incorporated that usage into several works.