The Primates were ape-like and they, more than any other Xindi species, resembled "traditional" humanoids. (ENT: "The Shipment") In common with Humans, they had skin that varied in complexion and most of the Primates had ten fingers. They had, like all Xindi, distinctive ridges on their cheek bones. They also had larger foreheads but no eyebrows. (ENT: "The Xindi")
The Primates were generally seen as fair, honest, and trustworthy by other Xindi. (ENT: "The Shipment") However, Xindi-Reptilians such as Dolim considered them not very resilient. (ENT: "Azati Prime")
They were also among the most intellectual species of Xindi and therefore one of their own, Degra, was assigned to develop the weapon to destroy Earth alongside the Xindi-Aquatics on Azati Prime. However, this intelligence also helped in convincing Degra that the Guardians' promises were empty.
|Aquatic • Arboreal • Avian • Insectoid • Primate • Reptilian|
In scripts for episodes ranging from season three premiere "The Xindi" to third season finale "Zero Hour", Xindi-Primates were commonly referred to as "Xindi-Humanoids". In the final draft script of "The Xindi", the species was even described as having "Xindi-humanoid features." Though the species was most often referred to as "Xindi-Primate" in canon, the species was also referred to as "Xindi-Humanoid" in "Proving Ground" and "The Council".
Following approval of the facial design for the Xindi-Reptilians, the makeup for the Xindi-Primates began to be designed. This turned out to be a fairly easy job, as it had already been decided that a common feature among the Xindi was to be spiky cheek-ridges. (Star Trek: Communicator issue 149, p. 56) The Xindi-Primate makeup ultimately consisted of a single prosthetic piece which spanned from the actor's mid-skull to their cheekbones.  The prosthetic was made from molded foam rubber which felt supple to touch and was subsequently painted for usage. 
The process of applying the Xindi-Primate makeup took about an hour and a half or two hours, whereas the removal of the makeup required forty-five to ninety minutes. (; BLANKMANinc.com: The Star Trek Interviews) Although the prosthetic appliance was extremely tight around the eyes, it was important that the relevant actors took the time to have the prosthetic applied. Remarked Tucker Smallwood, "That's not something you want to rush, no matter how late or tired you are. Not if you value your skin!" 
For the typical Primate head design, the hairline was raised, in an effort to make the aliens look significantly different from how extraterrestrials on the show commonly looked; Hair Designer Michael Moore also tried out various hairstyles for the Primates, in a series of concept sketches. "Somewhere in the work I just decided to try to bring the spots down lower and then blend it in with the rest. And so I sent that, along with the other drawings I had, to Rick [Berman] – and the envelope came back to me with a big check on that one," recalled Moore. "Sometimes when you send 12 drawings up, you think, 'I bet he's going to go with that one' – and I had that feeling with this one." (Star Trek: Communicator issue 149, p. 56) The actors playing Xindi-Primates regularly had their hair appropriately styled at the same time as their makeup was being put on. 
The costumes for the Xindi-Primates were inspired by centuries-old European uniforms, as Costume Designer Bob Blackman was intent on making them look like "something that was a little more ancient" than the costumes for the other Xindi species. "Caucasus shapes, from Eastern Europe/Russian shapes, is where I went to get this, 16th-17th-18th century – so it's kind of like Cossack coats," Blackman explained. "But that was just in order to find something that would give them some bulk, and some strength in how they looked." (Star Trek: Communicator issue 149, p. 56) The tiger-striped brown and gold jackets worn by the Primates had vinyl trim and a vinyl harness which culminated in a metal buckle.  
A pair of Xindi-Primate facial make-up applications was sold off on the It's A Wrap! sale and auction, included in a lot along with a group of Xindi-Reptilian make-up pieces. One of the Primate masks was painted and suspected to have been screen-used, whereas the other prosthetic was in its raw, unpainted form.  Two of the tiger-striped brown and gold coats worn by some of the Primates were additionally auctioned off on It's A Wrap! One of these, worn by the character Thalen, sold for US$354.99.   A costume worn by Degra was likewise sold at Christie's 40 Years of Star Trek: The Collection auction, in a lot including three other Xindi costumes. 
In the novelization of "The Expanse" and "The Xindi" (entitled The Expanse), the Primates are cited as having been the second Xindi species to have evolved a very high degree of intelligence, preceded by the Xindi-Aquatics. The book also establishes the Primates as having a pubis.
According to The Expanse, the Primates dominated most of the Xindi homeworld for eons, ruthlessly oppressing the other Xindi species (which they considered inferior) to the point where the others (particularly the Xindi-Reptilians and -Insectoids) developed a hatred of them that remained into the 22nd century. At that time, the Primates still controlled the majority of the planet but had (seemingly at least) abandoned the belief of themselves as superior Xindi. Also, the Primates' language had, following centuries of Primate domination, become the official language of their homeworld. Most Primate politicians had originally considered the fact that the other species were gaining power and increasing in number as a tragedy, believing the developments certainly could only lead to more racial wars akin to the one which had virtually torn the planet apart. An oft-stated Primate opinion held that the Xindi-Reptilians, -Insectoids and -Arboreals had very limited thoughts. The reason why the Primates were the Xindi species that developed the weapon is given as being that they were the most talented engineers.