Sigma, better known to his peers as Lifeform: Designation Sigma-Tourmaline-7-4-Baker, was sent to Earth to gather intelligence for the forthcoming invasion.
The aliens rightly figured that his small size and winsome large eyes (the smaller ones are less winsome, but then they can be scanning for intelligence while the big ones are distracting the humans) would enable him to easily infiltrate human society.
What the aliens didn’t realize was that once Sigma discovered the Furry Purry Thing (and chocolate chips), he decided life was pretty good here on Earth with the humans in charge. So he defected and is now a proud Earthling who loves snuggling his Furry Purry Thing…
…even if the Furry Purry Thing is not always so keen on the idea.