Just a traveling warrior at the service of anyone in need.
She is a traveler, but has homes in Lindon, in The Shire, and in Rivendell.
There is indeed one year missing from most tales, from the day when Túrin son of Húrin left Doriath, and until he joined the Gaurwaith; in that time, he met a wandering Nandorin maiden by the name of Edhellen, who reminded him of a lost, forgotten friend from his childhood. Amarthniel was born in those forests to them both, but soon after, her mother was murdered by some of the many groups of outlaws that dwelled around the area, when her father was not with them. Therefater she was raised by him, despite the burden of raising a child in the wild, for he could not leave her under someone else's care at the time. Then, as years passed, despite his attempts to send her away in order to keep her safe, and the secrecy that both kept of her identity and their kinship, in an attempt to keep Amarthniel safe from her father's curse, they were never truly apart.
The stories never tell of her, but she was always there, hidden, in the background; from the time in Amon Rûdh until Turambar's stay amongst the Men of Brethil in Ephel Brandir, later witnessing his impaled corpse in Cabed Naeramarth. Amarthniel then passed under the care of King Thingol and Queen Melian, meeting Húrin Thalion during his brief visit there, when he called her Ídhrambar, after her longing for her father and his ruined fate.
She served as a warrior in Menegroth, and she later met Dior Eluchíl and his family, who, being related to her, brought her some peace of mind until the fall of Doriath, when her inability to find and save Eluréd and Elurín consumed her in grief. In that time she met Maedhros, son of Fëanor.
Moving then in search of Elwing, she met Eärendil, her cousin, and again peace returned to her heart. But this did not last long, for soon the attack on the Havens of Sirion, and her belief on the death of Elwing and the captivity of her twin sons brought desperation back to her life. Later, after she found them under the care of Maglor, and witnessing the children's love for him, she remained with them, (developing, as well, love for the remaining sons of Fëanor), until the War of Wrath.
After these events were over, even though Morgoth had been defeated, Amarthniel knew that his influence on Arda yet lingered, and thus, he was not truly gone. So, despite Eönwë's message to follow into her father's destiny and die as a child of Man, she chose to remain as an Elf, until the last of Morgoth's designs were eradicated from the world; even though this choice meant that she would never see her father again.
SOME OOC INFO
I'm aware of what I appear to be doing with such an OC, but this is my homage to my own father, who recently passed away. This was the OC I used to play with him and his friends (all huge Tolkien nerds), when they all got together for some TTRPG sessions set in Tolkien's universe, for my dad most of the time played as Túrin Turambar. I want to leave a living note of those days. That's all.
So, I love you, dad. See you on the other side.
Any that would align themselves with Evil.
The star of Eärendil, which reminds her that she is not alone; the sea and sea-faring, despite the bitter memories it brings her, for it holds many joyful ones as well; The Shire, the simple life of its good folk, and the peace they bring to her.
Evil beings and persons, hipocrites, dragons (DESPISES them), and more than anything, Morgoth.
The promise and her own oath to remain on Middle-earth until the last remnants of Morgoth's designs are erased from Arda.