Louis Arpels (1886–1976) was the youngest of the Arpels brothers who joined their brother-in-law Alfred Van Cleef in the house at 22 place Vendôme. Van Cleef & Arpels dates his arrival to 1912; Wikipedia gives 1913. Inside the firm he was in charge of sales, while his brother Julien ran the business side.
In 1933 he married Hélène Ostrowska, a model for the House of Worth, born in Monte Carlo in 1907. He often asked her opinion on prototype jewels, and she became known for wearing jewellery in unconventional ways, with the catchphrase "Diamonds go with everything." In August 1940 the couple sailed on one of the last liners to leave for the United States before the end of the war, and in New York they helped establish the American branch of the business, which opened at 744 Fifth Avenue that year.
A lover of dance, Louis is credited by the house and by French Wikipedia with inspiring the ballerina and fairy clips made in New York in the 1940s. The first, the Little Winged Fairy clip later known as Spirit of Beauty, and the first ballerina clips date from about 1941, and the theme has remained one of the house's signatures; it was later taken up by his nephew Claude, whose friendship with George Balanchine led to the ballet Jewels in 1967.
The couple later became estranged; Hélène went on to found her own shoe business in New York, trading as Hélène Arpels from 1970. Louis Arpels died at his home near Paris on 20 March 1976, aged 89.
The house dates his arrival to 1912; Wikipedia gives 1913.