Elemmírë was a Vanyarin Elf, famed for his creation of a lament for the Darkening of Valinor, the Aldudénië. When Melkor assaulted the Two Trees, the Vanyar and other Elves of Valinor were celebrating a festival on Taniquetil, and so Elemmírë most likely witnessed the destruction of the Trees with his own eyes. The song he made to lament the death of the Two Trees was said to be known by all of the Eldar.
Note that the form of the word does not allow us to know whether Elemmírë was a male or a female Elf. This article, as well as other sources, consider Elemmírë a male.