Treecats obtain their name from the felinoid appearance of the head, but the body, in both movement and shape, more closely resembles a Terran ferret or other mustelid. Treecats possess six limbs: the true-hands in front, possessing 3 fingers and an opposable thumb, the hand-paws midway down the body, which can be used either as extra hands or stabilizing extra feet, and the rearmost true-feet. All six limbs possess curved retractile claws approximately 1cm in length, composed of a tooth-like mineral construction with an extremely sharp inner edge. Treecat tails are prehensile, with the ability to be carried either rolled into a tube for warmth, or flattened to exposed the gripping underside surface. 'Cats also possess secondary nerve centers near the midlimbs, thought to be related to psychic ability.
Treecats lack the vocal apparatus to speak other languages, but a modified version of sign language was eventually developed to allow them to speak to humans.
Treecats possess telepathic abilities with each other, and empathic ability among themselves and other sapients (such as humans). Mated pairs form a lifelong bond with their partner. Treecat ethics include a strong proscription against interfering in the mental processes of others, but the can serve as a supportive and stabilizing influence.
Treecats are also able to bond in a somewhat different fashion to humans. They are able to sense the "mind-glow" of humans, despite our being "mind-blind" (non-telepaths). This bond could be quite intense. The difference in treecat and human lifespans before the advent of life-extending prolong treatments led to tragic separation with the death of the human partner, as the 'cat might decide to suicide rather than live with the bond severed. The extension of human span to more closely match that of the 'cats remedied this, however. In a few cases, humans gained the ability to sense the emotions of other humans through their 'cat partner.
Due to their empathy, it is difficult to impossible to hide hostile intent from a treecat. Presenting a danger to a treecat's friends or adopted human will result in an immediate and likely lethal attack response. They can also detect those who have been psychologically modified, registering as a "wrongness" in the afflicted's mind-glow.
While treecats are generally carnivores and lack the ability to digest cellulose, they use the plant "purple thorn" to maintain their psychic ability. However, they discovered that Terran celery ("cluster stalk") is an even better "telepathy vitamin" and enjoy it immensely.
Treecats are organized into 12 known (formerly 13) clans, each clan governed by a group of Elders. Clans also possessed one or more "memory singers" (automatically Elders in their own right) who serve as "oral" historians of the clan. Memory singers possess exceptionally strong memory and telepathic powers, able to "replay" events from clan history with great detail.
Treecats are hunter-gatherers, possessing a roughly Stone Age level of tool use. They can create stone blades and tools, as well as cordage-based tools like nets. Their tech level is more based on lack of need than inability, as they are fully capable of learning the use of the advanced technology of humans.
Treecats are culturally cautious, tending to advance more slowly then the more reckless and headstrong humans. With the destruction of the Black Rock Clan, the 'cats were convinced of the need to expand offworld, lest they become collateral damage from human warfare. The first group of treecats to move offworld were the family of bonded 'cat Nimitz, who moved his family and certain others to planet Grayson. Whether this "Grayson Clan" will expand to a full new thirteenth clan remains to be seen.
- Lionheart (Climbs Quickly) - First treecat to bond to a human.
- Dianchect (Tree Dancer/Seeker of Dreams) - First Treecat bonded to a Manticoran sovereign.
- Nimitz (Laughs Brightly) - Bonded to Honor Harrington, founder of first treecat offworld colony.
- Sorrow Singer - Sole survivor of Black Rock Clan; proposed allying with humans to directly fight the Mesan Alignment.