Elmar Barthen was the proprietor of Barthen's Provisions, the largest trading post in Phandalin.
A friend of Elmar named Gundren Rockseeker believed he had located the Wave Echo Cave. Elmar was excited by the potential discovery, and Gundren had hired a new group of adventurers to bring his wagon full of equipment from Neverwinter to Phandalin, specifically to Barthen's store. Gundren let the proprietor know that he would meet him at the store before the adventurers, and that Barthen would agree to handover 10 gp each. However, Gundren never arrived.
Barthen was constantly harassed by the Redbrand Bandits, which made local trade more difficult. He was willing to share information with The Party, who wished to sort the Redbrands out.