As a recent college graduate holding down his first job in the 1980s, Barack Obama did not make much of an impression on someone who knew the ways of the world: His girlfriend’s landlord. “I can honestly say that I do not remember him,” said Ralph Allen, who owned the brownstone in Park Slope, Brooklyn, where Mr. Obama’s girlfriend at the time, Genevieve Cook, rented the top-floor apartment — an apartment Mr. Obama shared with her in late 1984. “Maybe we met while I was sitting out on the stoop one night. Who knows.”