Name: The Pareesans of the planet Pareesus in the Gallilius system.
Appearance: Pareesans in their natural, pureblood state are quite humanoid, though they can be characterized by longer arms (Around 6 inches, on average), slender necks, and pronounced shoulder blades. After a few generations of interbreeding on a given planet, however, they will be virtually indistinguishable from the native residents (assuming the residents are at least generally humanoid). A close inspection, however, will still reveal a cross-breed Pareesan to have similar traits to a pureblood: though they will be less pronounced. The quickest way to tell the difference is to examine the blood: A Pareesan's blood tends to have a slightly purplish hue when spread thinly enough.
Attitude: Pareesan culture is nomadic and exploratory. They seek to better their own genetics by planet hopping. At any planet with an intelligent species with genetics and phisiology close enough to their own, the Pareesans will drop off a number of individuals (Primarily male) dependent on the size of the population: A larger population means that more individuals will be left. These individuals are tasked with, essentially, procreating. How each Pareesan does this is up to him or her; some seek to find a lasting relationship to live out their life in relative peace, while some take on the role of a 'playboy', trying to spread their genes as far as possible. The alien nature of the Pareesans are, in general, a closely guarded secret, passed on only to a few select groups. This can lead to cult-like behavior, where Pareesans will gather together to, to put it bluntly, hang out. Because of this secrecy, a vast majority of Pareesans do not know that they are descended from an alien.
The Pareesans are what one might call 'slightly psychic', or 'empathic'. They are very good at reading emotion, but it is a very subtle power, and anyone with some psychic training will be able to tell when they are being read this way. This ability is used initially to facilitate the Pareesans' ability to attract a mate. The psychic ability tends to be diluted over the generations, however it never fully disappears. Because of this unique (and easily detectable) psychic imprint, when the Pareesan Convoy returns it is particularly easy to find natives who have descended from Pareesan blood, though generally there are several among the group privy to their alien nature who are prepared to leave with the Convoy. If no such individuals exist, occasionally there will be 'abductions', in which alien-descended people will have some DNA extracted, a few simple tests run upon, and will then be placed back where they came from. Those who choose to remain with the Convoy will live out the rest of their life in comfort as an esteemed member of society for the genetic services they have provided.
Technology: The Pareesan Convoy, the group of interstellar vessels that the core "pureblood" Pareesans travel in, is quite advanced; Despite its ever-growing size, the ships that they build are practically undetectable by most scanning methods. The technology aboard the ships is largely scientific, focused mainly on the development of new technologies in genetics, medicine, and space travel. The trademark of the Pareesans is their adaptability. With their technology, they constantly strive to improve their DNA strains to be more stable: offering longer lives, requiring less sustenance, improving learning capacity, and improving resilience to disease and other negative effects. They do this, generally, through their method of interbreeding with other planets' species. They do have a militia, though it is lightly armed and used only to hold back an attack long enough for the Convoy to escape.
The Pareesans left on a planet, however, are very often left with no such technology, as they must blend in as much as possible in order to properly pass on their genetics. Occasionally, an artifact will be left with them. Sometimes, this is seen as a 'ticket' when the Pareesans return, and they will target individuals who posses these artifacts for 'pick up' as it were.
Backstory: The residents of Pareesus very quickly formed a planet-wide coalition. Their species was small in number, and so gathering together and coexisting was the best way that they could survive on a very hostile, wild planet. Originally, they lived their lives underground, away from the majority of the dangers on the surface. In this environment of relative safety, they cooperated and switched their views to the sciences. Space travel became something of a fascination to them, as it offered an escape from their underground lifestyle. As such, it was a major focus of their endeavors. Others, however, decided to focus more on learning as much as they could about their own genetics and how their DNA worked, and on ways to improve their lives and, more importantly, their life spans.
Eventually, these two goals blended together and most of the Pareesans left the planet, a few million strong, and began to travel the galaxy. This is how their nomadic lifestyle began, and how they began collecting genetic information from various planets.
On Earth, the Pareesans landed in approximately 200 BC. Save for a few scattered groups around the world, no one really knew of the existence of these aliens. These groups tend to be small communities, meeting in secret under the guise of a book group or a fan club or whatever cover they have chosen. In these groups, they actually tend not to speak about their heritage, but rather they simply meet others to remind themselves that they are not alone.
Present: The Pareesan Convoy's goals have not changed in all this time since they began their travels. Since their arrival on Earth, their genetics have been spread so far that as much as 80% of the human race is at least distantly related to these progenitors. The few groups left on Earth who know the truth about themselves have different motives, though primarily they meet purely to speculate on when the Convoy may return. On Earth, it is highly unlikely that any 'pureblood' Pareesans remain, though technically it is a possibility. However, such an individual would have required a large amount of in-breeding, and as such would have experienced unknown genetic defects and modifications.
The date on which the Convoy is set to return is, in fact, in the year 2075, though the exact date is still unclear, depending on a large variety of factors.