The Arvonians (ar-VOH-nee-un) are a humanoid species similar in appearance to humans, but this appearance is deceptive. They are actually pseudo-mammals, with some features of Earth amphibians, such as extremely smooth skin that exudes a thin mucus that looks and functions similar to sweat. They have neither scales nor hair. Instead, Arvonians have a chameleon-like control over their skin pigmentation, which allows them to create patterns on their skin, either as decoration or to mimic features such as (for males) mustaches or beards. Since encountering the Terran race, the Arvonians have gone to great lengths to duplicate human physical appearance, to make it easier to engage in diplomacy, espionage, or even infiltration and seduction.

Arvonians of both sexes have a slender build, and tend to be extremely beautiful, particularly given their ability to change their appearance. Their society is matriarchal, so most of their starship captains and leaders are female, although male and female Arvonians are treated as equals. It was a female ruler, Queen Sasha, who ordered the building of the Supreme Understander, the sentient computer that runs all aspects of the Arvonian government. Known in the Arvonian language as "Lilene", (li-LAY-nay) the Supreme Understander is considered to be female, and rumored to be based on Queen Sasha's own brain patterns.

If this is true, then Queen Sasha was a crafty and manipulative ruler. Many of the Supreme Understander's schemes have directly led to war between the Arvonians and their neighboring races, including the USFP. She seems to be particularly curious about the Terrans, and often arranges with the Kraliens to set up listening posts and data relays along the USFP border so she can spy on the Terrans herself.

 The Royal Palace where Arvonians may get an audience with the Royal Family 
  or the Supreme Understander, if not willing to use the ultrawave link.

The Supreme Understander has also led to what is probably the most well-known trait of the Arvonian people, their almost fanatical reverence for computers. They refuse to use automated drones or torpedoes, instead they use piloted fighters. Their fighters are carried into battle by Light and Heavy Carriers.

It is easy to misunderstand this regard for computers, though. It is not that Arvonians consider their own lives disposable compared to a machine. It is that the pilot gets to turn around and come home. The torpedo does not, and the Arvonians regard this as a tragedy, almost as if one of their own babies or a puppy were on the missile. The fact that the torpedo isn't sentient isn't the issue, it's that it is being sacrificed for the greater good.

This actually carries forward into the Arvonians' regard for all life. They are particularly fond of space whales, regarding them as beautiful and going to great lengths to protect them. Torgoth, in particular, often hunt whales for trophies, and Arvonians will break off in the middle of battle and abandon their former allies if they see this happen. When it comes to combat, Arvonians are not ruthless, and will offer or accept surrender when lives are at stake. They are a proud people, however, and are willing to die for what they believe in.

A persistent rumor among non-Arvonians is that Arvonians can change gender through an unknown process. It is known that the Arvonians use drugs to enhance their skin pigmentation abilities, and even to form hair-like growths. They give birth and nurse their young, like mammals, and are sexually compatible with humans, even if no offspring can result from such a union. Arvonians tend to be extremely passionate, sensitive and nurturing. It is possible that the matriarchal bent in Arvonian society is simply a sign that Arvonians form a more intense bond with their mother than their father.

For information on the Arvonian language, see here.

From the Terran Stellar Naval Academy Tactical Manual:

The Arvonians are a humanoid race most remarkable for their chameleon-like skin pigmentation. They prefer to run curving patterns of very bright colors across their faces, giving them a dazzling appearance that most humans find attractive. Arvonian males and females have the same slender build. Their culture is matriarchal, so their space ship captains and fighter squadron leaders are often women. 

The Arvonian people regard their Royal Family fondly. A century ago the Arvonian race was united under the benevolent rule of Queen Sansha. As their culture spread to the stars governing became too complex for Sansha, so she decreed the construction of a super-intelligent master computer to assist her. That computer, whose name translates to “The Supreme Understander,” now runs the entire Arvonian government, deciding everything from war strategies to parking tickets. 

Most humans think of Arvonian society as a computer-controlled dystopia, yet the Arvonians adore The Supreme Understander. In fact, they revere all computers and refuse to put them in danger. Whenever a TSN torpedo or Torgoth drone is fired Arvonians weep for the weapon’s onboard computer callously sent on a one-way mission. Thus the Arvonians use manned fighters instead of automated missiles. 

Arvonians also love space whales. If they see you harm a space whale the Arvonians will fight to the death rather than allow you to live.

From Mike S.'s TSN Intelligence Manual "How to Spot an Arvonian Spy"

The Arvonian language has no adjectives; instead it has nouns with embedded to-be verbs. When presented with these two dialog options:

1. "We're taking hits to the forward shield!"
2. "We're taking hits to the shield that is forward!"

A native English speaker would naturally think of #1. A native Arvonian speaker would naturally think of #2.

TSN Intelligence Recommendation: if you suspect an Arvonian spy aboard your ship, listen for the crew member who forgets to use adjectives and instead uses nouns with to-be verbs. If your captain says "Sound the Alert that is Red!" then you'd better throw him in the brig.

Another aspect of being an Arvonian is reverence for computers and Artificial Intelligence. Arvonian culture has a revulsion toward hurting AI machines such as the guidance mechanisms on homing torpedoes and drones. This is learned behavior, not instinctive. Arvonians can choose to conceal their revulsion for the TSN launching homing torpedoes just as Americans choose to can conceal their revulsion for the Taliban forbidding women to go to school. In fact an Arvonian raised by humans might be totally cool with homing torpedoes.

TSN Intelligence Recommendation: If you suspect an Arvonian spy aboard your ship, watch for the crew member who sheds a tear every time you fire a homing torpedo.

Arvonians treat males and females exactly the same way. There is no such thing as an Arvonian cross-dresser because Arvonian fashion designers don't make men's clothes or women's clothes, they just make clothes. And Arvonian clothes are much nicer than human clothes . . . just ask any Arvonian.

The Arvonian language makes almost no distinction between genders. All of their personal pronouns and most of their titles and honorifics are genderless. In the Arvonian Royal Navy, you address a superior officer with a genderless honorific in the same way you and I address an emperor as "your Excellency" or a judge as "your honor."

In fact the only gendered titles that remain in the Arvonian language are the ancient terms for the Royal Family. They still have gendered words for their beloved Kings, Queens, Princes, Princesses, etc..

This means that Arvonians get confused by the TSN's gendered honorifics like sir, ma'am, Mister, Miss, or Ms. Sometimes they will just pick one at random without thinking.

TSN Intelligence Recommendation: if you suspect an Arvonian spy is on your ship, look for the crew member who calls Captain Horatio Hornblower "ma'am" or refers to Ensign Frank Pulver as "Miss Pulver."

Arvonians developed pigment change as a defense mechanism against predators. In a natural state, a frightened or angry Arvonian would automatically take on the coloring of his/her environment.

In modern society, it is unfashionable to lose control of your pigmentation. Arvonians have resolved this by developing various forms of martial arts. A student of one of these martial arts would learn to develop a fight face, a color pattern that is strong and intimidating. A fight face often has some characteristics of a fierce animal or supernatural demon.

Because of this, almost every Arvonian has the equivalent of a black belt in at least one form of martial arts. Some Arvonians know many forms of martial arts and can choose between many different fight faces if they become angry or frightened.

TSN Intelligence Recommendation: If you suspect an Arvonian spy on your ship, try to make him/her angry. If he/she is an Arvonian they may suddenly loose pigment control and take on the appearance of a hungry monster. TSN Intelligence cannot confirm rumors that some Arvonians have been trained to develop a fight face that looks like a normal human complexion.

Roleplaying and Cosplaying Suggestions:

As stated above, Arvonians look a lot like humans, and have been known to infiltrate TSN crews. However, crew exchanges have taken place during the periods of peace between the Terrans and Arvonians, and some Arvonians have gone rogue, either because they have been persecuted by the Supreme Understander for "variant behavior", or simply because they seek adventure. Thus, it's entirely possible an Arvonian could openly be a part of a TSN or even a Ximni starship crew.

Arvonians are intelligent, fun-loving people who enjoy wordplay and jokes. They have a well-deserved reputation for being smartasses and pranksters or, alternatively, almost insufferable divas.  Many other races feel that Arvonians, as a people, have a somewhat overdeveloped sense of the absurd. The art of subtle teasing and putdowns, called chogo Ta (Soul Tickle,) is a prized skill in cultured Arvonian society. Their culture uses sex as a bond reinforcement and has no taboos against casual recreational sex with strangers or family members or compatible other species or shapely inanimate objects, making their luxury package cruises quite popular among the more wealthy, adventuresome and uninhibited Terrans.  More traditional Terran (and especially Kralien) sexual prudery is a source of endless amusement for Arvonian comics. (Note, however, that Arvonians and Terrans are not genetically compatible)

They are passionate consumers of Terran popular culture, particularly the antique films and songs of the early 20th century which often amuse them into collapsed, giggling heaps on the floor. Recordings of Terran elephant and whale song move them to skin ripples, which are the Arvonian equivalent of tears. They usually despise Torgoths and avoid personal contact with them, even when their governments make pragmatic alliances, but they have a strangely reverential attitude towards Skaraans. Kraliens they consider dull and lifeless. 

Because of the close resemblance between Arvonians and humans, cosplaying an Arvonian is largely a matter of makeup. Facepaint in bright, garish colors, along with headdresses to hide your hair, or even accentuate the hair as if it were a wig or decoration would go a long way towards creating the proper look. Arvonian dress should be flashy and fashionable, if unisex. Note that Arvonians can use drugs or other enhancements to grow hair, and like their skin it will often appear in bright, unusual colors. Anything a punk rocker or Anime character would be seen in would fit.

Finally, keep in mind that since Arvonians can change their appearance fairly rapidly (in a matter of hours) if you get tired of the makeup, you can always change it, or even take it off. An Arvonian may decide to "return to human appearance" just because it is time to get to the business of running a starship, and quit fooling around.

See this post for Arvonian costume ideas. Most of the ideas in this section come from that thread, originally posted by Gryphon. (Also the author of the language page)