HERE BE SPOILERS!  Watch the series first if you care about that.  If not, trudge right along.



Dr. Eggman:
The star of the show with an IQ of 300.  He only appeared on Mobiearth very recently, and plans to conquer it.  His past and true nature are a mystery, and his vast powers make him a fearsome opponent.  Since he appeared to transform from Ninaj, could he be an aspect of that being?  First appeared in Return of Revenge (AKA Robotnik's Revenge).



Sonc:
Similar to a certain blue hedgehog we all know, Sonc is distinctly different from his counterpart, having the IQ of a doorknob and horrid luck.  He hates fat people, and attempted to kill Ninaj without provocation.  Sonc died upon contact with Dr. Eggman.  How he returned to life is as yet a mystery.  First appeared in Return of Revenge (AKA Robotnik's Revenge).



Ninaj:
Though he has a tendency to speak in all caps, Ninaj is a benevolent being who, at heart, has the best interests of the world in mind.  His powers are vast and great, to the extent that he may in fact be a god-like figure on Mobiearth, even at one point undoing the destruction of the entire planet.  He bears a striking resemblence to Dr. Eggman.  First appeared in Return of Revenge (AKA Robotnik's Revenge).



Dr. Wilight:
A mad scientist who describes himself as "evilgood", having good intentions but being unopposed to horrific deeds, believing the ends justify the means.  He hates Dr. Eggman and wishes to kill him.  He appeared to perish in Return of Revenge - Sidestroy B (his first appearence) but it's likely the wily doctor has a few tricks left up his sleeve.



Zerox:
The first Wilight robot to attempt to kill Eggman.  Zerox was armed with an unimpressive Buster weapon, but was annihilated before he had the chance to bring out anything more powerful.  First appeared in Return of Revenge - Sidestroy B.



Gigantor:
A giant robot who resembles Dr. Eggman.  Appears to be sentient.  Can be piloted.  Little else is disclosed at this point.  First appeared in Return of Revenge Christmas Celebration Animation.



Dancing Stick Men:
Random extras.  Basically townspeople with no bearing on the plot.  Have :/ expressions.  First appeared in Return of Revenge Christmas Celebration Animation.



Pac-Man:
He constantly asks if people know how his past is like.  There are few times anyone has actually paid attention to him.  First appeared in Return of Revenge Christmas Celebration Animation.



Blue Eggman:
A mysterious blue Eggman that appeared in the themesong.  He has yet to appear canonically, so it's uncertain as to what, if anything, he truly is.  First appeared in Return of Revenge Christmas Celebration Animation.



Big the Cat:
He's big, but smaller than Gigantor.  First appeared in Return of Revenge Christmas Celebration Animation.



???:
A mysterious demon-like being that appeared after Eggman's battle with Supersonc, which resulted in the destruction of Mobiearth.  First appeared in Return of Revenge Episode 3 or 4 or so - Trouble on Super Island!