The Second Heir

All of Middle Earth believed Aragorn son of Arathorn to be the sole heir of Gondor; just as there were nine members of the Fellowship of the Ring. However, every tale has pieces missing with each generation that passes. A key person is often left out of the story because she was not one of the original nine that left Elrond's house.
A half-elf named Maethriel has Isildur's blood running through her veins as well but has no knowledge of her lineage for her own safety. After Orcs destroy her village, she soon meets the fellowship. After fighting to earn their trust, she helps them on their journey to save Middle Earth, winning the heart of the elvish prince along the way.

(Updated and Edited version from Fanfiction.net, same author)