Edited from the Wikipedia Entry.
Planet of Origin: Kilrah, a harsh and arid world.
The Kilrathi are race of warlike, feline extraterrestrials. Their society depicted as an empire. Physically they are bipeds who strongly resemble big cats: they have leonine manes, but also have markings which distinguish their clan of origin.
The Kilrathi's ancestors were among the most fierce predators of their planet and, due to the rise of sapience, they quickly became the dominant species. Intelligence itself emerged at first as a means to enhance the Kilrathi natural hunting abilities and to preserve them from the dangerous environment of their planet; they not only started to build tools and traps to capture their prey, but also shelters where to take cover during the volcanic eruptions that often rained acidic ash on the surface of Kilrah. Due to their fierce territoriality and the growth of their communities, which required larger territories for hunting, the Kilrathi soon learned that creating weapons and traps to kill other members of their species was as easy as preparing new tools for hunting.
From that point on, Kilrathi history is an extended chronicle of brutal civil wars where technological discoveries played a key role in dictating which factions would rise and which would meet their demise. The advent of space-faring technology simply enlarged the sphere of expansion of the Kilrathi and transformed dozens of planets into battlegrounds. Such intra-species conflicts lasted until about two centuries before the period where the Wing Commander games take place; by then the Kilrathi were divided into two, huge empires who battled to the death to establish who would rule over the whole species. The leader of the winning side was acclaimed as the first Emperor of the Kilrathi after centuries of bloody wars. At this point it was only obvious that such a power hungry race would direct its desire for conquest towards other species living in the nearby regions of space. The Kilrathi conquered many worlds without difficulty, as not one of their enemies had the force or the technology to compete with them.
Kilrathi society is highly hierarchical: essentially the whole race is composed of an upper class of nobles (the Thrak'hra), and a lower class (the Kilra'hra). Thrak'hra and Kilra'hra are divided into hrai, the Kilrathi word for clans. There are eight Kilrathi noble clans whose members are part of the "Council of Eight", a political organization directly below the Emperor. Throughout history, these clans have made a special effort to preserve the purity of their blood-lines; they are the nar Caxki, nar Qarg, nar Kiranka, nar Ragitagha, nar Ki'ra, nar Kurutak, nar Sutaghi and nar Sihkag.
A Kilrathi's future, both as a warrior and a citizen, is determined by their social status from birth: only the nobles have a right to rule or command, and the common Kilrathi simply follow their leadership. The Kilra'hra clans swear allegiance to one of the noble clans and throughout the generations this servitude is passed on. Kilra'hra obey their masters in everything and are not allowed to do anything without explicit permission. As such, one of the most important concepts of the Kilrathi culture is that of honor: each Kilrathi must act so that they does not dishonor their race, master or their own clan. It is not uncommon for an individual who has fallen into disgrace to commit Zu'kara (ritual throat-slashing suicide): such failures could range from dishonorable behavior in combat to unintentionally insulting a superior.
The source for the belligerence and expansionist behavior, which are associated to the Kilrathi, has recently received an interesting new explanation. This is based on some hypothesis developed by research conducted during the Confederation clashes against the race codenamed "Nephilim". The "Tome of Sivar", an ancient Kilrathi relic, contains the so called "Prophecy" whose writing is attributed to the prophetess Kt'lan: it foretells the coming of a dark age for the Kilrathi (Kn'thrak), marked by the invasion from an unknown civilization (which is supposedly the Nephilim). According to Kilrathi lore, the Prophecy could be connected with an encounter between this alien civilization and the Kilrathi themselves during their ancient history. The invaders deemed the Kilrathi unworthy of fighting until they had risen to conquer the other races in their sector of galaxy. It is thus possible that the aliens have purposely or inadvertently forged the Kilrathi lust for conquest by triggering their sense of honor.
Kilrathi religion is based on the cult of the war deity Sivar, who is the only known Kilrathi god. Kilrathi females control all religious factions on Kilrah and direct all offerings and religious festivities. It's interesting to notice that most of the worshipping and celebration of Sivar takes place simply by fighting, hunting and offering part of the captured prey at one of the many temples dedicated to the war god. The most important religious ceremony known in Kilrathi culture is the Sivar-Eshrad ceremony which is celebrated every year and preferably performed on newly seized territory, a fact that drives most Kilrathi wars and has caused the confrontation with the Terran Confederation. This ceremony is of utmost importance since it ensures that the Kilrathi instinct to fight and kill is directed outward to enemies, rather than inward with the risk of sparking heated civil wars between the clans. Another ritual is the so called Pukcal (which translates as "ritual of atonement"): this ceremony is performed with the aim of confessing any private, minor dishonor to Sivar and receiving the god's forgiveness, thus avoiding mass suicide among Kilrathi who feel they have dishonored their clans.
According to the Kilrathi themselves, the destruction of their homeworld caused by the Temblor bomb, with the subsequent death of billions of their species, is considered as a huge form of "Pukcal" through which the entire race expiated the dishonor of fighting with duplicity at the "Battle of Earth" in 2668. Because of this belief Christopher Blair (the man responsible for delivering the Temblor bomb on Kilrah) sees a limited form of worship from the Kilrathi: his representation has been observed on the walls of a chapel aboard a Kilrathi vessel. Such a circumstance is probably related to a typical Kilrathi behavior which tends to idolize or sometimes demonize those enemies who proved their superiors in battle.
Being such a belligerent species, the Kilrathi never developed a specific taste for art in the same sense as a human may intend it: this is probably also a result of the Kilrathi history missing some real periods of peace where art and culture could flourish. What little the Kilrathi produced in form of orally transmitted or written art is obviously war oriented: myths of ancient Kilrathi warriors such as "Karga the Hero" or "Vorgath the Hunter" have been handed down next to songs, poems and reports of the deeds of past Kilrathi (emperors, nobles or warlords) who particularly distinguished themselves during some of the many battles the Kilrathi fought across the centuries. However, within the Kilrathi, such works are not intended as art, but rather as teachings, especially for the young who can learn what is expected from a true Kilrathi warrior. Another example of this kind of Kilrathi literature are the well known "Codices": this is a set of written principles with a profound value in Kilrathi culture, as they explain the Kilrathi concept of honor, and serve to pass on the militant culture to the next generation of Kilrathi.
Any other cultural aspect that in human civilization is usually related to art or aesthetics is mostly devoid of such concepts for the Kilrathi. Everything is built, produced or decorated according to strict rules of functionality. Kilrathi designers never developed an appreciation for order and symmetry, and therefore, their designs are frequently asymmetric and discordant by human standards. The imagery used is iconic and bold, oriented almost entirely towards glorifying previous battles and intimidating foes. For example dressings and warrior uniforms look unrefined, a peculiar mix of functionality and ornament, high-tech and primitive. Favorite colors for textile fabrics are reds, blacks and maroons and the decorations are almost exclusively symbols and insignia identifying the warrior's clan and family. Metallic textures and elements of personal armor (used mostly by nobles) lean toward gold, copper and blood-red colors.
Architecture is driven by these same principles of functionality and is adapted to the Kilrathi's peculiar taste for death and destruction. Kilrathi buildings bear evident resemblances to the primitive caves their ancestors used as lairs, since pillars and protrusions are often found rising from unlikely locations. Exposed machinery is common and ceilings are rarely enclosed, leaving cables and ductwork exposed. Details such as the dark lighting, high temperatures and low humidity favored by the Kilrathi complete the description of their constructions. Decorations are minimal and mostly represented by trophies of war such as pieces of captured or destroyed starships or the skulls of victims.
In spite of such primitive principles, Kilrathi technology is very advanced or at least on par with human technology. While their starships and fighters lack whatever could be considered as aesthetically appreciable to human observers, these vehicles are extremely efficient war machines, designed to be lethal. Kilrathi vessels are built with asymmetrical elements, protruding angles, weapons and engines placed in plain sight and often incorporate fang or claw-like curves and points, making them resemble ominous-looking blades or other melee weapons. This last detail is thought to be as much an expression of the Kilrathi visual taste as a form of psychological warfare aimed at intimidating their adversaries. Decorations are limited to ship markings indicating the clan responsible for the ship's construction. In combat the Kilrathi are strongly biased toward smaller numbers of powerful heavy fighters coupled with swarms of swift but inferior light fighters. The Thrak'hra lords pilot the heavy crafts while the lowborn Kilra'hra are assigned to fight in the light crafts. Despite this division, many Kilra'hra aces have emerged, some of whom have won acclaim with the nobility. Ace pilots are also authorized to fly customized fighters mounting different energy weapons or loadouts and can decorate their ships with personal markings.
Source: Wing COmmander Universe.