Highly intelligent beings, the Greys like to travel the universe in search of sciency stuff. Including biological specimens for complex reasons of their own. So it is that they have visited Earth, remote though it is. Earthlings are interesting.... if you're a freaky scientist with a hidden agenda that is... Anyway, they're small and a bit wobbly, but extremely bright, and they make some of the best tech stuff in the whole Galaxy.
The species known as 'humans' live on a minor unknown planet in a remote corner of the galaxy. Humans are virtually unknown in the 'Imperium', the great Galactic Empire. Harry seems to be the only one, though there may be others. Those that do know of them consider them to be weak, worthless beings of little consequence, primitive and ignorant.
Kind of fish faced half kangaroo humanoids, Icthysupials are famous all over the Galaxy for being sneaky, untrustworthy liars and conmen. Con-things I should say. That may appear harsh, and rather Itchysupial-ist, but unfortunately it is largely true. They've just evolved to be self-serving tricksters, always out to get one over on you. Mostly notorious cowards too. But not really evil as such. You know, like they won't try and eat you or anything. Just trick you into giving them stuff.
Tricrusians, or People of the Mound, as they like to call themselves, evolved on a planet with very heavy gravity, so they're stocky and very strong, with three powerful legs and a barrel shaped torso. They are covered in red fur (that can be dyed in different colours) and have faces that look a bit like cats to us humans. They have a quite complex societal structure. Their fur can photo-synthesise sunlight as well. They also hatch out of eggs...
The Leptira are a race of insectoid beings who are extremely carnivorous. They like nothing better than to wander the galaxy in their terrible Raptor spaceships looking for interesting beings to eat. They kind of resemble the praying mantis of Earth but six feet tall, with plasma rifles and super powerful spaceships. Sort of like evil cannibal pirates really. Whilst others collect gems, or knowledge, or wisdom, or alien art, the Leptira like to collect recipes. They are very interested in finding out what humans taste like, now that they've met one...