Lady Haleth of Brethil is in rarified company among mortals. Not only is she, alongside Tar-Ancalimė, Tar-Telperiėn, Tar-Vanimeldė, and Tar-Mķriel, one of only five known female leaders of Mannish nations (and in both of the latter two cases, their power was weilded by their husbands), she is also, with Emeldir the Manhearted and Eowyn of Rohan, one of only three mortal warrior-women in Tolkien's writings.
But how did she end up that way? The Second House of Men crossed into Beleriand around the year 311 of the First Age, but refused to move with the people of Bėor and Marach into Estolad, nor to continue westward. Instead, they settled in southern Thargelion, between the rivers Gelion and Ascar. The land was nominally owned by Caranthir, son of Feanor, but he lived far to the north, part of the Siege of Angband.
Haleth was born around 341, with her twin brother Haldar. Her people lived a spread-out existence, and for the first 35 years of Haleth's life, a peaceful one. At this point she was 'no-one special': the Haladin had no lord, so her father would simply have been a farmer, a hunter, a parent.
Except it isn't quite so simple. In 375, orcs swarmed out of the Blue Mountains to assault the Haladin, and Haldad, Haleth's father, was chosen to be their leader. At the southern tip of Thargelion, between two rivers, he built a stockade, and the orcs laid siege.
But how did Haldad know what to do? Where did Haleth learn the leadership skills she used when, after the death of both her father and her twin, she rallied the people and defended the fortress through another week of starvation, until Caranthir came to their rescue? The Edain would not have used such fortifications on their trek into the west, nor in their pastoral life in Thargelion. The Nandor of Ossiriand, to the south, would never make such forts.
There are only two possibilities. Haldad and his family could have travelled north and visited the Leaguer - but it is explicitly said that Caranthir ignored them, which would be difficult if they were paying him regular visits. More likely, a look at the map tells us that the dwarf-road from Nogrod and Belegost followed the north bank of the Ascar. Could Haleth have journeyed with her father to their ancient halls and high fortresses, and seen their skirmishes with the orcs? An architect who could design a stockade to hold off an entire army of orcs would be a wise choice for leader - and the gruff mannerisms of the dwarf-lords of the Blue Mountains would go down well with the wild woodsmen of the Haladin, whether weilded by Haldad or his daughter. And in after years, after Haleth dragged them through spider-infested valleys to Brethil, the Haladin famously went into battle armed with axes...
One further point: there were not, in fact, three Houses of the Edain. There were four - the fourth being the Drśedain, the Wild Men of the Wood who the Rohirrim named Woses. The distant kin of Ghān-buri-Ghān travelled west with the Haladin, and lived alongside them in Brethil - and in Thargelion. Haldad would surely have gathered these squat, dwarf-like Men into his fold when he built the stockade - and when he fell, with the best of his people's fighters, could Haleth's final bitter defence have been augmented by the disturbingly magical watch-stones of the Drūgs?