Role in the film
Nash was a convicted crime lord on death row who also happened to look identical to Ernest. This would allow him to come up with a plan to switch places with Ernest and evade death row, while having Ernest falsely imprisoned so as to fool the security guards into believing Nash was still at Dracup Maximum Security Prison.
As Ernest tried to reform the prisoners in his own eccentric (but well-meaning) way, Nash proceeded to try and life Ernest's life but was angered at the way people dismissed him and believed he was doing Ernest a favor when he started committing mean-spirited acts and criminal behavior that alienated him from Ernest's friends.
Ultimately Nash masterminded a plan to rob Ernest's own employers, who just so happened to be a rather prominent bank. Rubin Bartlett (Felix's deathrow inmate) grew attached to Ernest and felt guilt over letting an innocent man die just so Nash could further his own criminal lifestyle, which ultimately resulted in Ernest escaping prison. Rubin tries to stop Ernest from escaping the prison at its main gate, but Lyle tells him to spare Ernest. After knocking out Rubin, Lyle releases Ernest who tells him goodbye afterwards. Ernest engages Nash in a battle after arriving at the bank. After Ernest magnetically flies from the bank's roof and into the sky to take the bomb away from everybody and suffers its explosion, he falls back into the bank accidentally defeating Nash. Nash's ultimate fate was probably being arrested by the Howard County police officers and sent back to the jail.