King Neptune is somewhat narcissistic but he often keeps that trait to himself. During work hours, Neptune often takes a short 30 second break to make sure he did his task correctly. He only considers Firecracker a good friend.
As a Crewmate he does his tasks at his own pace because he had a head injury on the job some years past. Most of the others wonder if he was involved with some seriously terrible affairs on the Skeld or at Mira.
As the Impostor, Neptune shuts down the lights and uses the vents very cleanly. If only 2 Crewmates remains, he disrupts the reactor and "waits" for some assistance, he then kills whoever he deems worthy of death.