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IL-series Cylons typically take on the duties to maintain the non-military aspects of the Cylon Empire, typically commanding bases (The Young Lords) and basestars on important missions for the Empire (Lost Planet of the Gods, Part II, et al.) Unlike the Cylon Centurion, they have two eyes, similar in proportion to the eyes of humans, as well as a less-robotic monotone, yet harmonious male voice. They also express emotion, after a fashion, such as jealousy, power lust, and even laughter; reactions to dire situations also elicit responses, such as "Oh, no" (The Young Lords) and "Uh, oh" (The Living Legend, Part II).
The Imperious Leader is promoted from the ranks of the IL-group, as Lucifer notes to a Centurion that the current Imperious Leader was promoted over him, which he was unable to understand (Lost Planet of the Gods, Part II).
One uniform trait of the IL-series is their ability for logical thought, though this is at times subdued over the desire for power in the Cylon hierarchy. For instance, Lucifer believes that a major military victory under his command at Kobol would increase his influence amongst the Cylons (Lost Planet of the Gods, Part II).
There are also various versions of the IL-series Cylons. Lucifer notes to Baltar that Specter is an earlier IL-series model that is before "my time" and "rather limited in ability", though he does not elaborate for Baltar's benefit. One such possible trait of older versions of the IL-series are fabrications and the ability to spin a situation so that the Cylon remains in favor, as exhibited by Specter during his command of the Attila garrison.
Additionally, despite Lucifer's claim that jealousy is not programmed in the IL-series, Lucifer exhibits behaviors that run contrary to this claim; he attempts to discover flaws in Specter's command (such as the stockpiling of various munitions, despite his reports that Attila's inhabitants were terminated) and mutters "oh, felgercarb..." as Specter flatters Baltar during an update from the garrison (The Young Lords).
Lucifer is the IL-series Cylon that appears most frequently. Specter appears only once, commanding the remote garrison at Attila in the Omega sector (The Young Lords). Several unnamed IL-Series Cylons are seen milling about at the dedication ceremony on Gamoray (The Living Legend, Part II).