The Citadel Council:
Leaders: Citadel Council (represented by a Turian, Salarian, and a member of the UNSC)
Homeworld: The respective races' homeworlds, as well as the Citadel:
http://images1.wikia.nocookie.net/__cb20090207035233/masseffect/images/thumb/e/ed/MassEffect_2008-08-12_11-12-14-85.png/800px-MassEffect_2008-08-12_11-12-14-85.png
Name: The Citadel
Environment: Almost entirely cityscapes, though the Presidium mimics a temperate climate outside.
Regions:
The Presidium: The Presidium is a massive, park-like complex containing the offices of the various branches of the Council government, as well as the embassies of all the races represented in the Citadel. There are also shops, restaurants and recreational facilities, serving the wealthy residents who live and work throughout the sector. The Presidium is monitored by a Virtual Intelligence named Avina, and, unlike the Wards, incorporates an artificial 20-hour day schedule: lights are dimmed, and the holographic "sky" darkened, for 6 of the 20 hours, imitating the familiar day-night cycle.
Citadel Tower: The tower at the center of the Presidium houses the Council Chambers and the main traffic control for the station—any ship within a few thousand kilometers comes under the jurisdiction of Citadel Control. The Chambers themselves are not open to the public and access is only granted to officials and those with special clearance. They are beautifully furnished, with cherry trees and fountains, as well as numerous staircases leading up to the central platform where the Council convenes.
The Wards: The "arms" of the Citadel, known as the Wards, constitute the residential and commercial sectors of the station, housing millions of residents from many galactic species. Numerous skyscrapers rise from the superstructure, sealed against vacuum, as breathable atmosphere is only maintained to a height of approximately 7 meters. Unlike the Presidium, there is no artificial day-night cyle; as a consequence of this, commercial activity rarely ceases, and residents work and rest according to personal need. It is considered the cultural heart of the Citadel and Citadel-controlled space. The Wards are policed by the Citadel Security Services (C-Sec), whose offices and custody suites are located in the Lower Wards.
Dominant Species: Salarians, humans, Turians, Volus, Elcor, and Hanar.
Technology level: Incredibly advanced, using mass effect field technology among most of its residents.
Animal Life: None.
Territory: Citadel Space encompasses a large portion of the galaxy. At its center is the Serpent Nebula, containing the Citadel within, which acts as a major trade hub. It also contains the territory of the Turian Hierarchy, Salarian Union, Vol Protectorate, the Hanar Illuminated Primacy, the Elcor's Counts of Dekuuna, and UNSC space. Along it borders are the Attican Traverse, a largely uncolonized region of space dividing it from TGDA space, the Skyllian Verge, which divides it from the Terminus Systems and is the source of competition between the UNSC and Batarian Hegemony, and the remnants of Imperial space.
Armed Forces: The UNSC, the Turian Hierarchy, and the Salarian Union provide most of Citadel Space's defenses, primarily via C-sec and their impressive navies. The Citadel Defense Fleet is exceptionally powerful. The Special Tactics and Reconnaissance Group, or Spectres, are an elite unit of Special Operations troops who operate independently and largely free from Citadel law. They only answer to the Council itself. The Salarian Special Tasks Group also assists the Citadel Council in matters of espionage.
Philosophy:
The purpose of the Citadel Council is to unite a number of widely differing races to defend themselves from the staggering variety of dangers in the galaxy, as well as establish a major trade hub to support themselves. The Council does not believe any tales of the Reapers, nor do many of the Citadel's residents. They believe the Citadel to be a creation of an ancient race that existed before the Forerunners, left for other species to find and use (likely the race that created or found the Keepers). The Council has tried to establish relations with the Batarians, but they have refused to join the Citadel, mainly because of their bitter rivalry with the UNSC. Nevertheless, the Council is eager to spread their influence and invite other races into their fold.
Name: Keepers
Description:http://images4.wikia.nocookie.net/__cb20100914204252/masseffect/images/thumb/7/74/Keeper_at_Zakera_Ward_Level_27_Dock.PNG/793px-Keeper_at_Zakera_Ward_Level_27_Dock.PNG
Diet: None; don't eat.
Natural Abilities: Keepers are adept repairmen, and do nothing more than manage the Citadel's operation. They are completely mute and benign, and seem ignorant of everything around them. Attempts to capture a Keeper or take it into custody for study will cause the creature to undergo a sudden "self-destruct," with a form of acid being released internally. The affected Keeper literally melts into a puddle of proteins and minerals in less than a minute, preventing any serious research (as a result, the Citadel Council has made it illegal to interfere with Keepers in any way).
History: The Council simply accepts what the Keepers are, believing them to be created (they are techno-organic) by the race that also created the Citadel. In reality, they are mostly correct: the Reapers either indoctrinated or transformed the Keepers (similar to the Collectors) to be used as part of the Reaper genocide: when the Reapers' vanguard judged evolution of organic life to have advanced far enough, it would send a signal to the Keepers, compelling them to activate the Citadel's relay system and unleash the Reaper fleet upon the galaxy. However, the Forerunners, before being annihilated, managed to alter the signal receiver of the Keepers, who now effectively ignore the Reaper signal transmitted by their vanguard (which explains why Sovereign had to activate the Citadel manually). Though now useless to the Reapers, the Keepers still do their job at maintaining the Citadel, and its citizens seem to have no problem with it...