The Eye Guy and Mister Eyeball are fan-coined names for a character who makes a brief, but memorable, appearance in the comic. As the nicknames hint, all that is ever shown of this character is an eye, seen through a porthole-like peephole. This character is (or was) working with Ariadne Steelgarter and seems to have a connection to the Shining Brotherhood. And yes, it's possible that this individual is in fact an Eye Gal, and/or not entirely human.
As has been previously stated, all that is ever shown of this character is an eye, seen through a large peephole or small porthole. Some of the skin surrounding the eye is shown and it appears to be bluish-grey, although perhaps this is due to some property of the peephole and isn't the true complexion of the Eye Guy. There is also a bag under the eye and some apparent wrinkles beneath it.
In the Comic
Steelgarter summons the Eye Guy by knocking on the wall where the peephole is situated after she has defeated and killed (so she thinks) Violetta Mondarev, becoming badly injured in the process. From their conversation, Steelgarter is working with or for the Eye Guy. Possibly "for" because she feels the necessity of reporting to the Eye that she is leaving the dome housing the Queen's Society, but perhaps "with" because she shows no inclination to listen to his orders/requests. The Eye Guy expresses surprise and recommends against her leaving because the dome is currently on lockdown after the murder of Professor Tobias Homlomium. The Eye is also aware that Steelgarter tried and failed to steal a Skifandrian artifact from the room where Zeetha was staying in the dome with the goal of locating Skifander which was the cause of her fight with Violetta. Steelgarter ignores all of the Eye Guy's arguments and does manage to successfully leave the dome.
The true identity of the Eye Guy and the fate of this character when the dome is eventually blown up, during a complex and confusing battle, is unknown.
Connection to the Shining Brotherhood
The Eye Guy seems to have a connection to the Shining Brotherhood (also known as the Grey Hoods), although the evidence for this is perhaps somewhat tenuous. The only link is the phrase "The Great Work"; the Eye Guy tells Steelgarter to stay to help with whatever that is, although she completely disregards this request. The same phrase is later used by a member of the Brotherhood. What role the Eye Guy plays (or played) in the Shining Brotherhood is unknown.