Fellen Ni-Yaleii was a female Efrosian who lived in the 24th century. Following her escape from the Letau prison facility, she became one of Iliana Ghemor's lieutenants, although she believed her leader was Kira Nerys.

In the 2370s, Fellen operated as a gunrunner for the Maquis. She was arrested by the Cardassians and imprisoned on Letau. In 2375, the Letau facility was damaged after a Romulan starship was forced down by the Jem'Hadar, allowing Fellen and other prisoners to escape their cells. Before transporting to Harkoum with Ghemor, Shing-kur, Telal and Mazagalanthi, Fellen gathered weapons and set delayed plasma charges to destroy the Dominion transporter.

In 2377, Fellen, Telal, Mazagalanthi and Shing-kur were captured by the crew of the USS Defiant on Harkoum and brought to Deep Space 9 for interrogation. After Fellen learned that her leader was actually Iliana Ghemor and not Kira Nerys, Shing-kur activated an implants in Fellen's brain that released a neurotoxin that killed her in seconds. (DS9 novel: Fearful Symmetry)