Not much is known about Exebark's past besides the fact that he is a servant of Exeller.
Exebark makes his first debut into the series in Cream's route. If Cream catches up with the firefly, she finds dead corpses, and rushes to find someone for help. Exeller appears, and summons Exebark and Execun to capture her. They both are sent to run after the girls.
To be added...
Exebark is made by Exeller's blind bloodthirst, making him into a mindless animalistic beast with no purpose other than brutally slaughter his targets.
He has the same general color scheme as Exeller having blue fur. He has 4 legs with white paws. He also has tons of quills behind his back.
As of now all we know is that Exebark has great hearing, and the power to run faster than Cream can but he is blind.