Devan Shell is an evil tortoise mastermind and the main villain of Jazz Jackrabbit - parodying the classic "tortoise and the hare" rivalry of old: however unlike the slow-and-steady tortoise of legend Devan is a crazed inventor who has began conquering entire worlds in his quest to rule the universe.
Devan suppresses the native life forms of every world he has conquered and kidnaps the Princess Eva Earlong of Carrotus out of rage after the planet successfully fends off his invasion: thus Jazz Jackrabbit is called in to rescue the princess and put a stop the Devan's insane war-machine.
In Jazz Jackrabbit 2 Devan schemes to erase rabbits from history via time travel; he does this by stealing a huge diamond (in the picture above) from Eva's wedding ring just as she is about to be married to Jazz. Jazz and his brother Spaz travel to Devan's laboratory where they fight off his remote-controlled robot. Devan then uses a machine to send the bunnies through time and space, including the first three levels from the original game.
The two return to the present and journey to Hell, where they confront him in a final showdown. Despite Devan utilizing a new demonic form, they defeat him and restore him to normal. Spaz scares him, and Devan attempts to use the time machine to complete his plan, only for Jazz to intervene and knock the diamond off via a shot from his ray gun. The last we see of Devan is him poking his head out of the rubble, only for a rock to crush his head.
Had the series continued, in Jazz Jackrabbit 3 Devan would have used the machine to kidnap Eva and her children, forcing Jazz to travel to an alternate universe to rescue them.