Native to a world they only call "Homeworld", though officially called Kzin, the Kzinti are a highly aggressive, cat-like and warlike interstellar species.

Kzin are larger than equines, and a large male Kzin may be eight feet tall and weigh nearly 500 pounds. They are bipeds, standing erect on short legs. They have bright orange fur with individualized black markings, which most commonly appear on the face and hands. They are powerfully built, with thick limbs and torso.

Although mammalian they do not have a mammal-like rib cage. Instead they have a latticework of bone struts encasing their torso, connected by cartilage. The head is quite cat-like in appearance, although with a large cranial bulge. Their eyes are strangely colored, with round pupils instead of cat-like vertical ones. Their fan-like ears fold like parasols, and they have a naked, pink, rat-like tail. Kzin has a gravity of approximately 1.2 gee.

It seems that the Kzinti are on average less intelligent than Equines. Aggressive and territorial, Kzinti have much more need of "elbow room" than do Equines. A Kzin "grin" is not a smile- the Kzin is baring his teeth for an attack. When one Kzin challenges another, he screams and leaps. When time permits they frequently perform grooming, such as brushing their fur. The Heroes' Tongue, the Kzinti language, sounds like cats fighting.

Two males ready to protect a female

For the most part, female Kzinti are only semi-intelligent (or at least non-verbal) and are kept closely guarded in harems. They are never carried aboard Kzinti warships or intentionally exposed to the dangers of war. Kzinti females are in heat only when fertile, like most mammals. It appears likely that Kzinti engage in sex only for reproduction.

The Kzinti Empire is ruled by a feudal-style aristocracy. Those with high status have full names; those with low status have not earned a name, and are called by their profession. A moderate status earns a "partial name", for example "Chuft-Captain". Unlike Equine aristocrats, even a Kzin born to a high-status family must earn his name. According to the Puppeteers (who may have a biased perspective), Kzinti refuse to perform menial work or practice animal husbandry. Therefore they must have slaves to perform these tasks, "or be barbarians roaming the forests for meat". A "conquest leader", leading the conquest of a planet, is entitled to one percent of its wealth for life. Kzinti use base eight numbers, due to the fact that they have only eight fingers.