The Zarthans are a sapient species from the planet Zartha, which have been in war against the Kylothians for decades at least. In the 1970s a Zarthan delegation including their royal princess Lauranna went to Earth to seek protection for something called the Light of Zartha, which would be of vital importance for the protection of their homeworld against the Kylothians.
Believed by some to be some kind of artifact, it is revealed that the Light of Zartha is actually a person – Laura Vasquez – who is the daughter of Princess Lauranna and destined to become her people's leader. At one point, Agent K implies that Zarthans; or at least the Light of Zartha; may have unconscious influence over the weather ("it rains because you're sad"). This is apparently true as shown at the end of the film where she departs and comes to tears and it start to rain.
- It is unknown how they actually look, as all of them were seen disguised as humans. However, the fact that Laura was raised on Earth unaware of her alien ascendance would imply that what we see of the Zarthans is indeed their true appearance; a rare case within the Men in Black universe. On the other hand, the scene of Ben's death shows him being disintegrated and only the skin remaining, suggesting that he was wearing a human costume like most MiB aliens do. It's important to note that Ben himself is not stated to be a Zarthan and could be of another species. K's statement about Laura having her mother's eyes wouldn't hold much ground if all Zarthans are balloon beings.