Nebula of origin: Scientists are pretty sure it evolved from self-replicating electromagnetic formations within a nebula somewhere.
Species: The Hollow. There's only one.
One or two lines about appearance: A black cloak surrounding a roughly human-sized void of impenetrable darkness. Electromagnetic scans and exotic senses indicate the cloak is an artificial containment system within which lives a complex plasma field that resembles the neural paths of organic beings. In layman's terms, a generic 'energy being'. The cloak is made from fine metallic thread in a complex weave and may have been created by another species to enable the Hollow's adventures outside its nebula. It seems unlikely that a nebula-based entity could manipulate matter without help.
One or two lines about personality: According to evaluation by station xenopsychologists, the Hollow is an intelligence vastly exceeding the capabilities of the greatest organic geniuses, but operates on a timescale far slower. In theory, this means the Hollow can collate and interpret data so brilliantly that it is able to see the future. However, it totally lacks the basic comprehension of matter-based culture and cognition. In practice, this means the Hollow is annoyingly vague, uselessly prophetic and has the reaction time of a snail. Somehow, it remains able to be in places surprisingly quickly so long as nobody is looking.
It usually expresses itself only in lingering pauses and ominous, terse monotone, often stating something utterly vague to most but probably deeply profound if you observed the quantum foam as routinely as breathing. Since nobody else does except for quantum scientists, the Hollow remains vague at best.
Occupation: The Hollow first arrived on the station without warning. This caused a small security fuss, a short-lived resurgence in a doomsday cult (they do this every time a dark, cloaked alien species is discovered) and an inexplicable rise in fast-food sales. When queried, it introduced itself as A REPRESENTATIVE and when asked its purpose, said, OBSERVE. It did not answer whether this meant it was there to spy or that it was about to perform a magic trick perceptible only on some exotic spectrum. After a little more scrutiny, station security downgraded the Hollow's danger rating from 'herald of ancient galactic threat' to 'tourist'.
It does not appear to require any physical sustenance and spends most of its time wandering the corridors and saying weird things to strangers. It has been assigned quarters but as far as anyone knows, it has never been in them.
What your character was doing when the Alert went off:
The Hollow stood in the corridor. Lights flashed and sirens wailed. It shifted subtly, suggesting a motion of looking upward. It waited.
A jumpsuited man rushed past it, careening toward the safe zone beyond and brushing against the heavy cloak with a spark of static. "Dude! You better leg it before the doors seal!"