Robert Erskine

In the novel Evil Under the Sun, Sir Robert Erskine was an elderly rich gentleman and a late admire of Arlena Marshall. Hercule Poirot, viewing Arlena as a woman who men easily got tired off, surmises that Erskine died before he could have got tired of Arlena. He left her fifty thousand pounds in his will, much to the chagrin of his relatives. One of the relatives was married to a cousin of Emily Brewster. That money became the motive of the murder of Arlena.

Naming
Throughout the book the character is sometimes called Robert Erskine and sometimes Roger Erskine. It can be put down to the name being misremembered. Given that both Captain Marshall and Emily Brewster, who called Erskine Robert, have a personal connection to the man (Captain Marshall being married to Erskine's heir Arlena) they are likely most reliable to know the correct name.