This would normally be where I ask "how might one go about this?", but in truth, I have some ideas of how I COULD, if I decide that it isn't stupid/insane. My actual question here is "what should I bring?" Okay, so I like Eldar a lot, and they are one of my favorite 40k races, especially since I HATE all things Orky (occasionally, I do not have a sense of humor I am aware of), and a number of the other Koronus aliens are rather undefined, while Nids and Tau are blessedly absent. In this story, my crazy RT wants to bring an Eldar aboard, as a crew member, and the story even gives him a goofy option where he might propose this idea to a a Corsair captain he met, who could just maybe agree. What do I want out of this character, though? The ship has a good assortment troopers, who are pretty well equipped, on his coin, and both his ship, and its escort vessel have fine pilots (I favor the "look for exceptional individuals, when you can" approach, and am not sure if an Eldar would do much with a clunky Human craft. It's got some cool tech an Eldar might vaguely recognize, and knowledge of her people, but they don't warp travel like we do, so no "Eldar Navigator-analogue", and I doubt the corsairs have much for battle-psykers, especially that they'd allow to leave. Thus, I reach a point in my head where I know what I want, but not a good idea what i want out of it. This is story crap, so there aren't other people who need to accept it, work with it, nor assist in giving me ideas.

If you were going to add a xenos to your crew, especially one who isn't officially fleshed out in some capacity, like they did the Orks, the Kroot, or what have you, how would ask them to serve? It could be nice to have a good intermediary between peoples, if possible, and if they were going to interact often, but they need something else to do, with their other time.

Strictly in an advisor capacity...a proper Rogue Trader wouldn't let them touch the shiny bits.

And keep him under guard constantly. Eldar put Eldar interests first. Treated as an "honoured guest", with a permanent escort, "kindly" restricted to nonvital areas of the ship and likely there for the purposes of some common goal, upon completion of which he will leave. This does not mean the rogue trader, or the Eldar, will necessarily be unpleasant. They can be perfectly civil, even kind, but both parties would have their paranoia on overdrive and constantly be on their guard, even if mentioning it overtly would be "rude" or "undiplomatic".

OK, so.

First of all, bargaining with the Eldar is kinda hard, because they don't actually use the concept of money or trade. (They know how it works, but their society doesn't do it.) That said, there are only three good reasons an Eldar could join a Rogue Trader's crew.

1) The human has something the Eldar want, but not something they want so bad they'd kill to get it. This could be a fascinating piece of art, a remnant of a long-dead xenos civilisation that the Eldar are curious about etc.

2) The deal is beneficial for the Eldar. This could be, for example, because the RT's about to encounter a chaos artifact, and they want to secure it. Doesn't make it any less likely for the Eldar to backstab you (literally or otherwise) if they feel you're dangerous (because you actually secretly want the artifact, for example).

3) The Eldar seeks adventure. This would most likely be a ranger, as they are the kind of Eldar who want to see the wonders of the galaxy. However, if they run into something potentially dangerous for their brethen, they'll probably dissapear into the webway and go to warn their craftworld or something.

The Eldar do travel through the warp. Only if they have to, but they do. Their "navigators" are called Void Dreamers, they navigate using their crazy-OP divination powers, and they also perform the duties of "corsair battle psykers" as you called them.

If you want information about the Eldar Corsairs, they're described in Imperial Armour vol. 11 (Doom of Mymeara). It's mostly about their warhost (and it's mostly 40k rules), but there is a little bit of fluff.

We actually had an Eldar PC on our ship for a little while. (The player departed) The excuse I used was that of a farseer's prediction to the effect of: "Sometime in the near (for eldar) future, this human will be in a situation to prevent the destruction of many eldar. Make sure he survives and is in the right position to do this." I was actually referring to the Warpstorm trilogy as well as a custom metaplot that I was working on, both of which could be seen as "preventing the death of many eldar".

There were no pretense that they were anything but allies of convenience, but that was enough to keep the group together. She ended up having very little to do with Void Combat, instead acting as a second Arch-Militant, shuttle pilot and source of Lore: Koronus & Lore: Xeno.

So, yeah, as Errant said, it's mostly a kind of Advisor. Unless, in your case as a captain, he decides to bring his whole ship, and act as an advisor/escort.

My group has an eldar corsair as part of their crew. There was a deal where she would work for the Rogue Trader for five years if they handed over something her father (an influential captain in a corsair fleet) wanted, but didn't want to possibly spend eldar lives to get, and she thought of it as an adventure.

She's a preternaturally skilled pilot (up to PC levels) and a stealthy assassin (teleportation pack and psychic invisibiliy), and even has her own stealth bomber (which seems to be somehow semi-sentient, might have something to do with those gems on the console). She's also happy to mess around with those crude human ships since she's doesn't need to worry about damaging them (they handle like a pregnant Neechua, but it's fun to smash them into stuff).

So long as none of the filthy monkeys bothered her enough for her to gut them everybody's happy. There was this one ensigne Kirk that wouldn't stop pestering her... Was. She made an unpleasant example of him and the lesser crew have more or less left her alone since. Maybe happy is the wrong word. Crew morale has suffered a little given that there's a xeno on board. Fortunately they mostly don't see her unless something interesting is happening.

Hilariously the Rogue Trader is out of commission in a stasis vault for the forseable future, which means that technically none of the PC's can give her orders since the deal was that she would serve him. They haven't found out yet though since they haven't asked/told her to do anything she wouldn't like to do anyway.

a stealthy assassin (teleportation pack and psychic invisibiliy)

If you gave her a Warp Spider's Warp Jump Generator, I'd just like you to know that pretty much every eldar other than a warp spider or autarch would never, ever use one of those. You know, getting caught by daemons and having your soul captured by Slaanesh for eternity of exteme torture is just not worth it.

So the question is what to do with said Eldar PC/NPC, and not how they came to be there? Well, depending on how they came to be there, it really depends on what they can do / want to do.

Scenario Uno: PC Craftworld Eldars were given a divination of the future prior to meeting the PC Humans. PC Humans and Eldar then meet, and fate was entwined since then. The PC Eldars left their people in less than neutral terms, and have since been traveling with the PC Humans in order to discover what the divination truly meant.

Scenario Dos: NPC Exodite Eldar’s planet and race was saved by PC RT, crew, and dynasty out of sheer accident. This female agreed, with the consent of the Lords of her world, to travel the stars with the RT to explore, until such a time as she deemed necessary to depart and return.

As for what they do, well, they are specialists in subterfuge. The two PC’s decided they would pilot void fighter crafts and hit and raid attacks on enemy forces, while the NPC decided she would escort the RT and crew on exploring worlds and discovering lost Eldar relics / technology. So it really depends on what your PC’s and you want from the game.

a stealthy assassin (teleportation pack and psychic invisibiliy)

If you gave her a Warp Spider's Warp Jump Generator, I'd just like you to know that pretty much every eldar other than a warp spider or autarch would never, ever use one of those. You know, getting caught by daemons and having your soul captured by Slaanesh for eternity of exteme torture is just not worth it.

Already taken into account.

A: She was formerly on the path of the warp spider and has had her armor warded the same way normal warp spider armour is.

B: The thing her father wanted? It's a rune attached to a witchblade the PC's picked up on the Dread Pearl. The rune either allows for much safer travel through the warp, or it allows her to push for much longer jumps than a warp spider's jump pack would normally allow (albeit with increased risk). Having been lost on the Dread Pearl for thousands of years her father is hoping to figure out how to reproduce it the way humans hope to reproduce archeotech. Imagining boarding parties that simply teleport accross, or through blast doors, or past defense hard points, etc. With something like that he might very well become the leader of the corsair fleet.

Until she has a chance to turn it over (I have plans for later on) she's happy to use it herself.

Getting her aboard is actually rather easy, since the RT knows a Corsair Captain, and they have a bit of a friendly relationship. The problem in my head, though, is who would she loan out. The Void Dreamer is a rather cool idea, in my mind, and Aedan doesn't have a cool battle-psyker among his entourage. His AT is less that-oriented, and one of the few other men of importance aboard, though with next to no "back history", and I assume his Navigator is similar, as I haven't fleshed that one out yet. My super-hot nav character is on an unrelated ship, as she's an older invention than the Qel-Drake Dynasty. Still, seems a rare commodity to give out, a corsair psyker.

An idea I did just have, and the Void Dreamer is even a doable option for it is this. So, I postulated earlier a terrible idea (that I happen to like) where a splinter-faction of Eldar bring a force of Tau to the Expanse, and the Tau take the opportunity to learn about the Eldar, and find a new territory to exploit. Many of the Eldar do not support this, but the Tau are, seemingly, uniquely incapable of making use of this benefit without willing participation from the Eldar, so my universe Harlequins are taking a watch and see approach. His corsair colleague, Captain Nyvetha Cirdanvae of the Mist of Lileath , might take the opportunity to warn him of the Tau intrusion, and loan him a Void Dreamer to locate the Tau, and also keep her in the loop. Whether she warns him of the Eldar involvement with the Tau is iffy, but she suggests Aedan inform the Calixan Battlegroup, hoping that they will assist in stopping the problem. Since Tau vessels don't seem to take part in boarding actions, the likelihood of the rogue Eldar aboard would probably go unnoticed, so the Imperium wouldn't seek further reprisals against all the Eldar. Whether these "treasonous" Eldar die with their Tau allies, or escape in the ensuing chaos is mostly irrelevant. Aedan would, of course, realize he's being a bit used, but would also use the tool given, while possible.

How about a Eldar Harlequin? Those guys will do anything to further their cause, and tricking an RT to do their bidding suits them.

Well from a meta perspective I'd use them as an occasionally fault intermediary between the crew and their peoples who either have a common goal with the RT or a bit of wanderlust. Like if you had a gang of cowboys and one of them was a Native. I'd imagine it'd be useful the majority of the time if you wanted to resolve situation diplomatically, and occasionally it'd make the situation worse.

*gun shots and the screams of the dying*

RT:"I thought you said you knew these guys!"

Xeno:" I do, they just liked me a bit less than I remembered!"

*explosion sound*

My players would likely use them as an adviser, and intermediary with some xenos group that aren't always killed on sight as a rule. Depending on the type of xeno, and whether or not it was a PC. It might be integrated into the crew as another bodyguard, schemer, assassin, tech guy, or even a leader of a small contingent of merc xenos. My PCs are loyalists and don't particularly like xenos, but their players enjoy interacting with them. So like many RTs they're willing to do trade, and set up semi consistent working relations with ones they think successfully manipulate. I can see them pointing a freeloota in the direction of the thickest fighting, sending off a DEldar to assassinate one of their rivals, hatching a plan with a vanilla Eldar, or most anything else. Plus of course the occasional RP of interacting with such a creature on a semi regular basis.

When it comes to specifics for your Eldar you got a pretty decent idea. A semi friendly corsair with a plan, loans someone to act as an adviser and possible muscle to deal with something that is in their mutual interests. But of course, there's always a bit more under the surface, and nothing is totally heads up.