An Etymological Dictionary of the German Language/Annotated/wer
wer, pronoun, ‘who,’ from Middle High German and Old High German wër (earlier hwër), interrogative pronoun; the nominative singular preserves the r as a representative of old s; compare Gothic hwas, ‘who,’ also Anglo-Saxon hwâ, English who. The Teutonic stem of the interrogative pronoun was hwa-, hwe-, from Aryan ko, ke, which is found in non-Teutonic in Latin quo-d, Greek πότερος, (κότερος), Lithuanian and Sanscrit kas, ‘who.’ Hence wann, weder, welch, wenn, wie, and wo. Further details belong to grammar.