Mariella was younger than Aaramoarne by 40 seconds, and the two resembled each other. When the Uruk Hai took over Minas Tirith when Mariella was seven, she was taken with Arwen to a dungeon. She watched Lortz kill Arwen with a whip right in front of her own eyes.
When she was 14, Mariella escaped the wrath of Lortz and ran to Minas Tirith. She found Legolas and the two fell in love for a period of time. Soon she and Legolas were married, and like Aaramoarne, she was full elf too, so she was immortal unless she was killed.
When Aaramoarne died, Mariella fell into deep depression, and never attended the ceremony. She and Legolas ran away together, to help Mariella forget, and soon, she had two children: Alfana, and Aaramoarne.
In the 21st age, Mariella impaled herself in the chest with a blade so she would die. She did. Two weeks later, she had her ceremony.