They were a large and physically powerful people with an average height of over 2.5 meters and weighed over 150 kilograms. (TNG - Intelligence Gathering comic: "A Matter of Dates"; FASA RPG module: The Orions: Book of Common Knowledge)
The Kaylar were noted for their aggression and were capable of a battle-frenzy that made them hard to stop with non-lethal weapons. (TNG - Intelligence Gathering comic: "A Matter of Dates"; FASA RPG module: The Orions: Book of Common Knowledge)
The Kaylar of Rigel VII were rated D-plus on the Richter Scale of Culture (ST references: Star Trek Maps, The Worlds of the Federation), known as primitive and savage, while their civilization was limited to towns, some agriculture. (FASA RPG module: The Orions: Book of Common Knowledge)
They were strongly territorial and xenophobic, and by the late 23rd century there had never been a successful contact that didn't end in bloodshed. The Kaylar preferred their solitude. (FASA RPG module: The Orions: Book of Common Knowledge; ST reference: The Worlds of the Federation)Their most advanced weapons were catapults, swords, spears (FASA RPG module: The Orions: Book of Common Knowledge) and other primitive hand weapons (Decipher RPG module: Worlds). Kaylar warriors and construction workers mid-24th century wielded large swords and axes in addition to energy weapons. (TNG - Intelligence Gathering comic: "A Matter of Dates")
They were expert in the ways of Rigel VII's powerful and erratic tides, however, and many were skilled mariners. They could sail across the dangerous oceans on ships that were huge in size but quite light, long, narrow, and shallow-bottomed. They were also known for skills in construction. (FASA RPG module: The Orions: Book of Common Knowledge; TNG - Intelligence Gathering comic: "A Matter of Dates")
The Kaylar were suspected to be originally native to Rigel V, but were taken as mercenaries and slaves and settled on Rigel II, IV, and VII, where they quickly returned to nomadic life-styles once their masters' empires collapsed. (Decipher RPG module: Worlds)
In the 2250s, when Rigel VII sought to join the United Federation of Planets, the Kaylar faced being disbanded as Rigel attempted to move its society forward from its warlike routes. The Zemtar fortress, a traditional Kaylar barracks, was to be the site of Rigel VII's admission ceremony. (EV comic: "Our Dearest Blood")
However, Rigel VII did not join the Federation due to a Kaylar coup on the eve of the planet's admission. The Kaylar attacks lead to the deaths of three crewpersons from the USS Enterprise and injuries to seven others. The incident shocked much of Rigellian society, spurring them to continue their reformations, to remove the more barbaric elements of their society with the aim of eventually reclaiming its place in the Federation. (EV comics: "Our Dearest Blood", "Nor Iron Bars a Cage")
Shortly after the coup, the memory of the Kaylar attack was used by the Talosians when they held Captain Christopher Pike of the Enterprise captive. To help draw Pike into the illusion his fellow captive, Vina, purposefully overplayed her role as damsel-in-distress to keep Pike from rationalizing the experience, too distracted defending her from the attacking Kaylar. (TOS episode: "The Cage"; TOS novel: Burning Dreams)
- Star Charts describes Pike's encounter as the first contact with the species and the world being quarantined pending development of warp drive.
The Rigelians believed that the Kaylar would destroy the dam if their demands weren't met, and prepared to abandon the colony. As Votar was a strategically important location to the Federation, Starfleet sent the USS Enterprise-D to mediate the dispute. Early attempts at diplomacy and a non-lethal assault led by Worf were met with resistance from the Kaylar as the Rigelians had convinced them the Federation would side with them and force the Kaylar out. Meanwhile, the Kaylar had no intention of destroying that which they'd built. Once these facts came to light, Worf was able to clear the waters and bring both sides to the negotiation table to find an amicable resolution to their dispute. (TNG comic: "A Matter of Dates")
There have been differing depictions of Kalar or Kaylar over a number of sources. The ST references: Star Trek Maps, The Worlds of the Federation, Star Charts describe the "Kalar" or "Kaylar" as the dominant species of their homeworld, but describe them as primitive and pre-industrial, and have Rigel VII be an avoided and quarantined world (likely by the Prime Directive) after failed first contacts. The Decipher RPG module: Worlds follows suit, but places on several worlds of the Rigel system, having them be the descendents of slaves and mercenaries deposited there by ancient masters (by context, these are likely to be of the Orion Empires).
On the other hand, the EV comic: "Our Dearest Blood" makes the Kaylar merely a sub-culture of the more advanced Rigelians, prospective Federation members at the time others place first contact. Meanwhile, the TNG comic: "A Matter of Dates", set a century later, has them as a somewhat civilized warrior people in their own right, employed in Federation colony construction, indicating a century of advancement. It also depicts Kaylar of much more typically humanoid sizes than in other appearances. Taking these two sources together, this perhaps suggests that they re-merged with humanoid Rigellians by the mid-24th century, or that the name "Kaylar" has spread to encompass both species.