I'm going to borrow a species from a book that I like. Allow me to introduce the Gentians.
Gentians are superficially very similar looking to humans, having the same essential body shape, but they differ from humans in a few key ways. The main one is that Gentians attain a biological age of 25 at the age of 3, then live precisely one hundred more years in the exact same health, dying rapidly at age 103. The second major difference is that there are only ever at most one million adult Gentians in existence at any given time. Gentians long ago lost the ability to reproduce naturally.
Every 100 years, all one million Gentians gather in a secret location, consolidate their memories in one massive pool, and then clone the next million Gentians. The ancestor Gentians have a huge end of life party as the new generation grows, working together to sort and prune the memories, and then the memories are copied to the next generation of Gentians to restart the cycle.
Several generations Gentian Cycles have given each Gentian a broad knowledge base in many topics. Gentians trade mostly in knowledge - it is their greatest asset, and what they value most. The breadth and quality of memories a Gentian brings back is what ensures their inclusion in the next generation. Boring Gentians effectively die, as none of their memories live on. Gentians who make the most interesting contributions are effectively immortal.
Physically Gentians are like 25 year old humans, in peak condition, as though they were Olympians of any given sport. Their mastery over biological processes gives them liberty to look like almost any humans, though most favor larger eyes, larger ears, and smaller frames, making them look sprite-like. Gentians have very morphic physiology, however. In the span of about a month they could change themselves to look like any human. They also have engineered themselves for survivability in a hostile universe. They don't need to breath, and can sustain themselves on nearly any source of energy, carbon and oxygen. Their bones have the strength of titanium while not being heavier than an ordinary human's, and their joints are made of a material which allows deformation under great stress, but always (eventually) return to their previous configuration. They die (as do most things) to high energy weapons and explosives, though conventional ways of killing a person can only, at best, disable them for a time.
Gentians, among races that know of them, have a reputation of being something like genies. Help is always rewarded many times over, while harm is disproportionately punished. Gentians try to foster their reputation by promoting the idea that finding a Gentian in trouble is a lucky event, like winning the lottery - help them, and afterwards, your life will be dramatically changed for the better. Harm a Gentian and The Worst Thing You Can Imagine will happen. Part of the duties of a Gentian is to subtly spread this rumor wherever they go, for the protection and well-being of future travelling Gentians. This meme-packet obviously contains how to recognize a Gentian (and spot imposters) and how to make it known that you are a potential ally once you've found one. The material is adapted to each particular culture.
Afinlo is a Gentian of about 50, who joined the Lancers mainly to gain access to many different species and worlds. His main interest is interpersonal relationships, how they are expressed differently among different cultures, how those relationships can change as situations change. He's 4ft high, kind of a blueish grey skintone and blue spikey hair. He wears a perpetual, disarming grin. He can be a bit of a prankster - he loves understanding people by pushing their buttons and seeing how they react. He'll defend himself and others if necessary, but abhors most violence. Killing any sapient creature removes a potential source of knowledge from the universe. Though, on balance, a mass murderer destroys much more knowledge than they produce themselves, so he'd kill one to save many. Afinlo also shuns most of the high technology of his kind, preferring to integrate more closely with the species he interacts with. Gentians have found that simple awe of their technology can be a barrier to learning about a species as it is. Not all Gentians feel the same (variances in opinion and temperament are deliberately introduced so that each Gentian goes off to do something different), but Afinlo feels that introducing high technology to the people he's studying destroys the traits that he's trying to study. So he uses only native technology he can find on whatever host world he ends up on.