Halmir was the lord of the People of Haleth at the time of the Dagor Bragollach ("Battle of Sudden Flame"). After that battle, and the capture of Minas Tirith, Orcs began to roam unhindered across the lands of Beleriand. Halmir called on the aid of King Thingol, who sent many stout Sindarin warriors under the command of Beleg to his aid. Together, Halmir and Beleg destroyed a legion of invading Orcs, and gave the southern lands a respite from the defeats in the north. Halmir was succeeded as lord of the Haladin by his son, Haldir.
Although not mentioned, it's possible that Halmir was son, or at least kinsman, of Haldan.