Xindi-Reptilian faces were scaly and highly textured, including many small wrinkles and ridges. The Reptilians had yellow to brown-colored scales with multiple spikes coming out of their heads and little tusks, one on either side of the face, protruding from close to the mouth. They had yellow eyes with slitted vertical pupils, and were cold blooded. Like other Xindi, they had a characteristic facial feature, involving a small indentation and a series of little bumps on their cheek bones. (ENT: "The Xindi", et al.) Reptilians' brains had neural pathways that were virtually identical to those of Xindi-Primate brains. (ENT: "Stratagem")
Xindi-Reptilians were also stronger and had greater stamina than Humans. For example, they were not affected by phase-pistols set on stun and survived direct hits from MACO particle weapons without suffering any immediately noticeable ill effects from such weapons, apart from being pushed slightly back. (ENT: "Rajiin", "Carpenter Street")
Reptilians were perceived as being untrustworthy and impatient by the other Xindi species. (ENT: "The Shipment") A common Reptilian saying was "Patience is for the dead." Nevertheless, the Reptilians were usually the first choice for when force was needed. (ENT: "Azati Prime")
With their easily provoked and opinionated nature, Xindi-Insectoids often sided with the Reptilians on certain issues. (ENT: "The Xindi", et al.) They sometimes even shared ships with each other. Reptilian soldiers were surgically implanted with a "suicide gland" that secreted a neurotoxin if they were captured. (ENT: "Rajiin")
The Reptilians were the species of Xindi that seemed to be most interested in destroying the planet Earth. The Reptilians lived out a militaristic lifestyle and hoped to ultimately rule over all the other species of Xindi. (ENT: "Rajiin")
Reptilian weapons, such as biorifles and seekers, used techno-larvae. Their biorifles worked on a rotating modulation that self-destructed when accessed by other species. (ENT: "The Shipment") Reptilians additionally used organic grenades fired from a grenade launcher which could be attached to a bio-rifle (see Xindi technology). (ENT: "Rajiin")
|Aquatic • Arboreal • Avian • Insectoid • Primate • Reptilian|
Of all the Xindi, the Reptilians were meant to look especially different from other aliens on Star Trek: Enterprise. Regarding the intentions for the Reptilians, Makeup Supervisor Michael Westmore noted, "[It] wouldn't just be a pullover head and scales – maybe a few little spikes, horny-toadish and nasty." (Star Trek: Communicator issue 149, p. 54)
Michael Westmore has repeatedly referred to the black spikes ascending from the Reptilians' heads as "porcupine quills," presumably because his idea for the spikes was influenced by the coat of a porcupine. (Star Trek: Communicator issue 149, p. 54; ; Star Trek: The Official Starships Collection, issue 103, p. 16) He admitted, "I've always wanted to do a character that had porcupine quills coming out of its head – that's why we twisted them, bent them; it's not just spikes."
Many concept sketches for the Xindi-Reptilians were done by Michael Westmore, before the design was whittled down. (Star Trek: Communicator issue 149, p. 54) In the final draft script for ENT Season 2 finale "The Expanse", the species was described as "terrifying-looking", with "animalistic features". Though "The Expanse" was the first appearance of a Xindi-Reptilian, a few modifications were made to the species thereafter. These included the addition of the bumpy facial feature common to all Xindi. (Star Trek: Communicator issue 149, p. 54)
Representing each of the Xindi-Reptilians' faces involved the use of a delicate, full-facial latex prosthetic.  The appliance specifically covered the neck and the entire front of the head.  The prosthetic was extensively airbrushed to create skin texture, shadows, and definition. The small tusks protruding from near the mouth and the black spikes extending from the head were made of resin.  The Reptilian makeup also incorporated pointed teeth which, compared to Ferengi teeth, were slightly longer and meant to look more menacing. Contact lenses were worn by the actors who played Reptilians, which is how their eyes were made to seem lizard-like.
The first time Reptilian hands had to be represented in an Enterprise episode, the series' makeup department glued scales onto the relevant actors' hands and gave them long fingernails. (Star Trek: Communicator issue 149, p. 54) In fact, prosthetic pieces were crafted out of foam rubber for both the tops of the hands and the palms. All the pieces were then painted for on-screen usage and the palm applications were designed to represent scales.  When Reptilians started being involved in stunt sequences, the make-up approach was deemed no longer viable, so the members of the species were portrayed as wearing gloves. This implied that Reptilian nails could retract, to fit inside the gloves, and extend. Michael Westmore remarked, "It should be interesting to see, at some point, all of a sudden the nails retract as they put a glove on."
The complexion of Reptilian skin also changed during the third season of Enterprise. The first three Reptilians to be shown on-screen all had much the same "neutral brownish tones," in the words of Michael Westmore. He cited the Reptilians, later in the season, as varying with a mix of green, brown, and red. (Star Trek: Communicator issue 149, p. 54)
In the final draft script of "The Expanse", the Xindi-Reptilian (in the first scripted description of a member of the species) was scripted to have "heavy armor".
After the theatrical release of Star Trek Nemesis had run its course, Costume Designer Robert Blackman decided to create the costumes for the Xindi-Reptilians, who were about to make their on-screen debut in ENT, by reusing Reman costumes from the film. They were made from a polyurethane stretch fabric that Blackman had already used in the creation of Starfleet EV suits for ENT. (Star Trek: Costumes: Five Decades of Fashion from the Final Frontier, p. 221) He later explained, "The idea was to reuse the Nemesis Reman foot soldiers [suits] but make them be entirely different." Originally under the impression that he would be required to create an enormous quantity of Reptilian uniforms under the ever-present time pressure, Blackman came up with the idea of altering the costumes with "cages". (Star Trek: Communicator issue 149, p. 55) "I saw a picture of a cage and thought that would be a good way to do it," he said. This was obviously a relatively cheap way of modifying the uniforms. Broad cages of wire armor were therefore added for the Reptilians' reuse of the costumes. Explained Blackman, "I used aluminium tubing as a frame, and made the rest out of vinyl hose." (Star Trek: Costumes: Five Decades of Fashion from the Final Frontier, p. 221) The alterations to the costumes included the removal of breastplates which had been part of the Reman configuration of the suits and making them into two pieces rather than a single piece, which meant they were easy for the actors to get into and were consequently more user-friendly for episodic usage. (Star Trek: Communicator issue 149, p. 55; Star Trek: Costumes: Five Decades of Fashion from the Final Frontier, p. 221)
The wire cages were extremely delicate, so a great deal of care had to be taken to avoid damaging them. ("Countdown" text commentary, ENT Season 3 DVD) Despite this, Bob Blackman thought they were indeed a good addition to the costumes. "We didn't think about them doing stunts in those, and the first time they got rough in them we just held our breath – but they came through just fine." The meshes also echoed the wiry quality of the spikes above the head. "And for all of that – Michael and I didn't really talk about any of that, the quills. So when you look at the quills and you look at the cages and go, 'Well – how did that happen?!' – I don't know how that happened!" (Star Trek: Communicator issue 149, p. 55)
As well as being portrayed by live actors and stuntmen, Xindi-Reptilians were also depicted with a mannequin (in the case of the Xindi-Reptilian corpse in "The Expanse") and computer-generated figures (such as in the first shot of the Xindi weapon's interior, in "Countdown"). (; "Countdown" text commentary, ENT Season 3 DVD)
The look of the Xindi-Reptilians influenced the CGI design of the Xindi-Insectoids and -Aquatics, as well as helping to make the task of designing the Primates easy. (Star Trek: Communicator issue 149, pp. 51, 52 & 56) Also, the faceplate from the Xindi-Reptilian make-up design went on to be reused; it was kit-bashed with other prosthetic pieces, to represent an unnamed Coalition of Planets dignitary in the two-parter "Terra Prime" and "Demons". 
The Xindi-Reptilians gained the approval of Archer actor Scott Bakula, who enthused, "I love the Reptilians... the visual that [the art department] created with the Reptilians I think is spectacular." (Star Trek Magazine issue 113, p. 7) Dolim actor Scott MacDonald also liked the Reptilians, commenting, "There were several extras playing Xindi soldiers, so it was pretty cool, me walking around with my troops."
In a 2010 interview, Michael Westmore cited the Reptilians as one of "two things I loved doing on Enterprise." He went on to refer to the species as "fascinating" and further remarked, "The heads, with the porcupine quills coming out of them, were gorgeous."  Consulting Producer David A. Goodman regarded the Reptilian make-up as "amazing" and "remarkable," an opinion which Director David Livingston agreed with. ("Impulse" audio commentary, ENT Season 3 Blu-ray) Regarding the Reptilian uniforms, Co-Executive Producer Chris Black admitted, "I love those costumes." Executive Story Editor André Bormanis stated, "It made sense in the context of the civilization we were trying to portray here [....] You can see these guys have a very distinctive look and that's a costume style that we've never seen before." Black agreed but he and Bormanis also joked that the Reptilians might use their uniforms, with all the tubing therein, to distil alcohol, concocting Xindi-Reptilian moonshine. Bormanis jested, "It's how they get through the day," and Black concurred, "Exactly. They have little jars at their waist that it's just slowly dripping into." ("Countdown" audio commentary, ENT Season 3 Blu-ray special features)
When asked to cite one of the Xindi species as the one he believed to be the most complex and disruptive in the Xindi Council, Jannar actor Rick Worthy initially responded with uncertainty as to which was the most complex but then stated, "Perhaps the Reptilians. They're warhawks, really. They're bullheaded, and it's difficult for them to see others' point of view." 
Several Xindi-Reptilian make-up appliances were sold off on the It's A Wrap! sale and auction. One such piece was a painted, heavily detailed face application. One of its head spikes had come loose from the prosthetic, owing to time and use of the mask, but the application sold for US$786.  A group of Reptilian prosthetics was included in the auction too, in a lot additionally consisting of two Xindi-Primate masks. The Reptilian portion was comprised of a full-facial application, a pair of hand top coverings and a pair of palm coverings. All these pieces were unpainted except for one of the palm coverings, which had been painted but retained the tear-away foam rubber which exists during casting.  Another lot consisted of a facial application and a chest and shoulder prosthetic piece. Both of these were painted and the mask was known to have been worn by Scott MacDonald.  Two other Reptilian masks were sold at the auction, one of which fetched US$760.  Yet another lot presented at the auction consisted of the breast piece from "The Council" and the facial prosthetic used for the aforementioned unnamed dignitary in "Terra Prime" and "Demons". The latter item was originally misidentified as a Xindi-Reptilian mask worn by Scott MacDonald. The eBay entry for this lot ended early, due to the error being stated in the listing. The mistake caused much confusion and resulted in MacDonald himself trying to ensure the listing was corrected. 
In the novelization of "The Expanse" and "The Xindi" (entitled The Expanse), the Xindi-Reptilians are characterized as being extremely tall, with a darting forked tongue, a vestigial tail, a cloaca and a penchant "for sudden, deadly strikes." The Reptilians' scales covering the face and arms are characterized as occasionally reflecting light by glinting like dull jewels, such as amber, ruby and emerald. The Reptilian central nervous system is also described, involving a brain stem, a basal ganglia and a spinal cord. Reptilian facial expressions are said to be hard to read, to the point of the book at one point describing the facial expressions as having "all the animation of a lizard sunning itself upon a rock."
The Expanse describes the Reptilians as having a pronounced hatred of the Xindi-Primates. According to Primate doctrine, the Reptilians thought with their spinal cords, their intelligence having evolved from only their brain stems and basal ganglia. As of the 22nd century, the Reptilian warrior class was swiftly gaining power. Purportedly, Reptilian skin was softer than it seemed. A current event in Reptilian culture was that the warrior who piloted the Xindi probe orphaned multiple Reptilian children, who were later presented with some lavish donations from the Xindi-Insectoids.