Devan Shell is an evil tortoise mastermind and the main villain of the Jazz Jackrabbit series - 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 starts his intergalactic crusade after reading the "Tortoise and the Hare" fable, getting disgusted by the attitude of the hare in the tale. Because of this he decides to conquer the entire universe and destroy all rabbits, believing them to be evil, maniacal creatures. He suppresses the native life forms of every world he has conquered but the resistance force of planet Carrotus successfully fights back his forces, so in order to make them surrender he kidnaps the princess Eva Earlong. During one of his intergalactic travels Jazz learns of the kidnapping and decides to gather information about Devan to rescue the princess, stealing weapons from one of Devan's stockpiles and traveling to planet Diamondus, where he begins his mission.
After liberating the planets Diamondus, Tubelectric and Medivo, Jazz learns that Devan is building battleships in a highly advanced part of the galaxy to destroy Carrotus. He stops by Letni, Devan's planet-sized supercomputer, wrecking the place before pursuing Devan's forces on Technoir and Orbitus. After liberating these planets Jazz receives a distress call from Eva, informing him that Devan was regrouping his forces on Sector-X, where he built his personal Mega Airbase. Jazz defeats Devan and destroys the base, but Devan escapes with the princess while Jazz crashes on a secluded area of the galaxy. Knowing that Devan was cloning his turtle troops with his Gene Machine, Jazz steals a spacecraft and escapes, pursuing Devan's forces and liberating more planets under his influence, finally reaching Devan's gigantic twin battleships. With both battleships destroyed, Devan sends a remote-controlled giant robot clone of Jazz to fight the hero while he escapes. Though Devan flees, Jazz destroys the evil robot and finds Eva, rescuing her.
Despite his previous defeat, Devan returns with a new scheme to erase rabbits from history via time travel; he does this by stealing a huge diamond from Eva's wedding ring just as she is about to be married to Jazz. The Queen throws Jazz into the castle's dungeon for his failure to stop Devan, but Eva sends Jazz's brother Spaz to release him so that they could go after Devan and stop his plans. Jazz and Spaz escape the dungeon and the enraged Queen, traveling to Devan's laboratory where they fight off his remote-controlled robot. After the robot is destroyed Devan uses the time machine to send the bunnies through time and space, ending on the first three planets from the original game.
The two return to the present and journey to Hell, where they confront Devan 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, destroying the machine in the process. 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.