Elrohir (Sindarin for Elf-knight, literally Elf-horse-lord) is the son of Elrond and Celebrían, the younger twin brother of Elladan and brother to Arwen. They were born Third Age 130, which makes them about 2,888 years old. The twins rarely part but fight wickedness together, fuelled by the memories of their mother's torment and that she left them early for the West. Elrohir and Elladan are so lookalike that it is hard to tell them apart, both dark-haired and grey-eyed. He can be distinguished from his brother by the colours of his clothing: his cloak is white with blue trim.
Many centuries later, Celebrían went on a journey into the south to visit her own mother, Galadriel, in the land of Lothlórien. In the Redhorn Pass, she was captured by Orcs, and she was tortured in their dens. Elrohir rode with his brother to rescue her, but by the time they reached her she had received a poisonous wound. Though their father healed her, she would not remain in Middle-earth, and sailed into the West the following year. After this loss, Elrohir and Elladan were filled with hatred of the Orcs
The twins often scouted or carried messages for their father, and also fought alongside the Rangers of the North. In the year T.A. 2,933, they rode against Orcs with Arathorn, Chieftain of the Dúnedain. Arathorn was slain in that encounter when an Orc-arrow pierced his eye, so Elrohir and Elladan took his young son, Aragorn, to be fostered by their father. Aragorn became good friends with the brothers and often adventured alongside them. They remained in contact with Aragorn in his guise as "Strider" and continue to support him.
Never will Elrohir or Elladan hesitate nor exit a quest, rather Elrohir will say: "From the North we came with this purpose, and from Elrond our father we brought this very counsel. We will not turn back."
It remains to be seen whether Elrohir will chose mortality, living with the Men of the new age, or if he will depart for the Uttermost West along with his dear father, and his mother before them. For the time being, he will dwell at Rivendell.
Elrohir (The Trollshaws)
Later, Elrohir and Elladan can be found at Thorenhad, the camp they established in the Bruinen Gorges in the midst of the Trollshaws. This is the base for the scouts who patrol the region, for Rangers and friendly visitors, and for many other purposes. Here Elrohir gives directions and organises the upcoming journey of the Ring-bearer.
Later, they briefly return to the Last Homely House to receive a mission from their father: the gathering of the Grey Company. They afterwards return to Thorenhad to begin this mission. Once the Grey Company has assembled in Rivendell, the Sons of Elrond can temporarily be found at Elrond's Stables. Elrohir is worried about his father's curious and foreboding words, but determined to stand firm against the Enemy.
-  Intro (High Elf): Into Darkness Falls a Star
-  The Sons of Elrond
-  A Scout in Nan Tornaeth
-  Return to Thorenhad
-  A Road Seldom Travelled
-  House of the Master
-  Down from the Mountains?
-  The Ford of Bruinen - Skirmish
- [...] Mission: The Further Adventures of Bilbo Baggins, Chapter 1
-  Volume I, Book 4, Chapter 2: The Missing Rider
-  Volume III, Book 1, Prologue, Instance: Messages from Lórien - Solo only
-  Volume III, Book 1, Chapter 1: Gallant as Elf-lords
-  Volume III, Book 1, Chapter 2: Candaith's Hidden Cache
-  Volume III, Book 2, Chapter 2: The Grey Company in Rivendell
-  Volume III, Book 2, Instance: As Near as May Be - Solo only
Elrohir (Ered Luin)
Together with Elladan he has arrived to the area to explore the background of a curious dream their father told and he wanted to scout the tunnels for signs. Meeting him gives evidence of his wisdom and his superior expertise in herb-lore.
-  Intro (Dwarf/Elf): A Putrid Spring
-  Intro (Dwarf/Elf): Medicine of the Elves
-  Intro (Dwarf/Elf): To Evade the Impending Doom
-  Intro (Dwarf/Elf), Instance: Skorgrím's Tomb
-  Prologue (High Elf): Reflections of the Past
-  Volume III, Book 4, A Full History - Elf
-  Volume III, Book 4, Instance: The Rescue
-  Interlude: Many Worries
Elrohir (Central Gondor)
-  Retaking Pelargir
Elrohir (March of the King)
-  Friend or Foe
Elrohir (The Wastes)
-  Chapter 1: Sauron's Hour