Eln was a Unit under the control of Akimus Makassar.
Biography
Eln's psychic projection appeared to Rose Tyler asking for help before Makassar regained control and began attacking Rose before disappearing.
Eln was one of the Units that tried bringing the Ninth Doctor and Rose under Makassar's command when the Doctor created a version of the Powell Estate to hide from them. When she called out for the Unit that asked for help in the crowd, Rose tricked the Units into turning and fighting him whilst the Doctor escaped.
The Doctor freed Eln from Makassar's control by placing another Unit's mask on Makassar's face, creating a loop. After this, Eln formally introduced himself and thanked the Doctor and Rose. (PROSE: The Masks of Makassar)