Agatha Christie Wiki

In the 1922 novel The Secret Adversary, Julius P Hersheimmer is a wealthy American man who inherited millions from his father. After that he thought of his relatives and wanted to help them. He had an aunt Jane Hersheimmer who married a poor school teacher Amos Finn. Julius's father did not approve of the marriage and he broke off with his sister Jane thereafter.

Julius employed someone to search for the Finns and he discovered that they had both died after moving to the west of the United States. But they had left a daughter Jane Finn, Julius's cousin. Jane had had gone missing since 1915. Julius helps Tommy and Tuppence Beresford to find the latter. He had already previously contacted several investigating agencies throughout Europe--including Scotland Yard- to find his cousin.

Hersheimmer had only one photo of his cousin which he gave to an investigating officer without realizing that the officer was actually Mr Brown.

Julius also proposes to Tuppence Beresford (née Cowley at the time) but the latter declines.