In Tolkien's fictional world, Eönwë was the the banner-bearer and the herald of Manwë, and Chief of the Maiar along with Ilmarë.
When the appeal of Eärendil reached the shores of Aman, it was Eönwë who first greeted him. When Manwë decided to heed the appeal, Eönwë was sent to Middle-Earth to fight the War of Wrath, leading the Vanyar.
When Morgoth the Black Foe was defeated, Eönwë took the two remaining Silmarils and held them for safekeeping. But at last the two remaining Sons of Feanor took them and fled, yet Eönwë did not let them be slain.