Emotion

"He checked his self-esteem level: it was absolute zero. Excellent. At least that was still normal."

Kryten changes a lot through the show. By season 8, he's definitely not the same droid that Lister and the crew picked up from the doomed Nova 5. Lister taught Kryten how to rebell when Rimmer started bossing him around. Then Kryten took a trip on a space bike and crashed into a planet. Lister tried to rebuild Kryten, but his personality was never really the same as it once was. And from there, Kryten started to change into a mechanoid with a personality all his own. He's since learned many emotions and human traits, many of which he tells Red Dwarf's psychiatrist of. Kryten tells him that he's learned pomposity, happiness, surprise, sadness, fear, lying, and anger with rudimentary, mindless violence. He also notes that he has yet to master ambivilance.

In episode 15, Polymorph, the Red Dwarf meets up with a creature known as a Polymorph. Polymorphs can change themselves into any shape, so they can be very difficult to spot. They feed off emotions, and often turn into something that will provoke someone's most strongest trait into its highest point, which the Pollymorph then feeds off of by sticking this tentacle/tube/tunk-like thingy to their forehead. When Kryten is attacked by the Pollymorph, the emotion he loses is guilt. As a result, he becomes very mean and insults and puts down the rest of the crew. In the book Better Than Life, though, Kryten gets a lot more nasty, and kills Talkie Toaster (a talking toaster that Lister bought that Holly tries to use for company). Gulit is one of Kryten's most felt emotions, and it effects him a lot. When the Pollymorph is destroyed in episode 15 and the crew's emotions are returned to them, Kryten feels so guilty for having acted so mean that he decides to kill himself. Lister stops him, but it's something that Kryten's felt compelled to do several times due to his guilt.
A really, really funny quote from when Kryten loses a leg in the book Last Human displays a pretty good example of how Kryten thinks:

"He gazed down at his leg again. Who would want a mechanoid who was physically imperfect? There was no option: he would offer to terminate himself. He checked his self-esteem level: it was absolute zero. Excellent. At least that was still normal. Yes, when it was convenient he would wipe his hard drive and close down his programme. It was the only solution. No, He was being absurd. Idiotic and ridiculous. How could he possibly terminate himself? Who on earth would tidy up his body parts? Terminating himself would just make the place untidy. Logically, therefore, he had to continue living."

In episode 37, Tikka to Ride, Lister replaces Kryten's regular head with one of his replacement heads, and took away his guilt chip. As a result, Kryten stopped caring about pretty much anything. He didn't bother at all with being super-polite. Among other things, he smoked, drank alcohool, and said things like, "You bet your ass!" VERY funny. ;-)

In episode 46, Back in the Red Part 2, the resurrected crew of the Red Dwarf decides that Kryten should have all of his emotions erased so that he can return to being the emotionless, desireless droid he once was. The crew is sucsessful in doing this (not before Kryten puts up a rather, um...strange fight), but not for long. Kryten starts remembering certain things when he sees Lister, the Cat, and Kochanski in a room, and then he quickly returns to "normal".

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