Jack B Nimble has not been seen since System Failure, when Jormungand ripped the encryption apart, and it went around and started 'saving' Crash victims as e-ghosts. Its true nature and what has become of it are complete mysteries. Though, since neither Big D nor Captain Chaos could break the decryption, I've always had the idea that it is either the work of a genius like Leonardo, or a Resonance construct of some sort.