After the Witch-king of Angmar destroyed the Northern Kingdom, the surviving Dúnedain of Arnor retreated to the Angle south of Rivendell, while smaller populations settled in far western Eriador. The last of the Dúnedain of Arnor had become a dwindled and wandering people known as the Rangers of the North. There, among the secretive nomadic folk, a pair of twins was born. Boy and a girl. "My Sun and Moon" said their father, Elren, a famous Ranger who was known as the Storm Giant among their people. From birth Elrenniel and her brother Belegorn were raised in constant journeys, in the wilderness and in the midst of the many dangers lurking for the Faithful.

From early childhood, they were taught the history of their people, elven language and tales of the former glory of the lost Numenor. Although both of the Elren children were skilled horse raiders, Elrenniel was by far the most skilled, and probably would have carried a sword if her father had allowed it. Belegorn states in jokes that Elrenniel was "half a horse herself".  Later, with some help from her twin, she has learned a few defensive skills, and practices nearly daily to keep her skills sharp. Though she is not a skilled fighter, she can defend herself. She grew up to be a proud woman with great diginity, but she was strong, and free of mind, and filled with the desire of knowledge. When in company with others she would often sit still listening to their words, and watching their gestures and the movements of their faces. Her relationship with her father was a close one, and according to Elrenniel, Elren cherished her best of his children. Later, as a competent and intelligent woman, she was also considered to be a trusted confidant and adviser by her father. When Elrenniel was seventeen, her mother died in childbirth, along with her unborn son. Elren told his daughter that she had to be the lady of the house in her mother's stead.

When she came of age, her father arranged a marriage for her with one of the Rangers. She didn't know her fiancé very well, it was supposed to be an arranged marriage, but she agreed to the engagement out of a sense of duty. In any event, the life she was to have with him ended abruptly when he and her father was captured and slain by Trolls in the Coldfells during one of their missions.

Belegorn, the Ranger himself at this time, took his sister to Rivendell, where she lived for four years, while her twin set off on long journeys. Yet as Belegorn grew in renown and strength, earning the respect of Elves and Men in many lands, Elrenniel seems to have retreated into her grief. She left Rivendell and returned to her own people in the desolate wilds of the north, where the Rangers were a people in hiding.

There came a day when a messenger from Forodwaith fared there, and he bore word to Elrenniel from Belegorn. And the slender, dark-haired lady looked upon the scroll from her brave brother, and she was glad.

Yet when she broke the seal, his words were grim:  

Elrenniel,

There is a monster somewhere within the Evendim, Emyn Uial, and he preys upon the innocent and unprotected. I do not speak here of the goblins or orc, but instead of a butchering fiend. Sister of mine, should aught happen to me, seek out Dôlguzagar the Black Numenorean, for he is the evil I hunt.

Belegorn

Elrenniel felt a cold hand grip her heart, and to her came a vision of her brother's face, a steely glint in his grey eyes. Two years passed, and no word came to her from Belegor. Where he was, what he did, was unknown.

And then came that dreadful night. One morning in the spring of that year, she awoke heavy after unquiet sleep about her brother, and a cloud lay on her thoughts that day. Her heart filled with fear. She came to Bree to find the other Rangers and ask them to help find her brother.