my interpretation of Jay
Here's my interpretation of the Jay character (and the movie).
Many of Jay's thought processes are typical of Asperger Syndrome, a mild form of autism that causes difficulty in understanding social situations but does not affect overall intelligence.
Jay being unable to tell whether girls are just being polite or flirting with him... his pausing to contemplate the literal meaning of idioms that others take for granted... the fact that his classmates often try to trick him into believing absurdities... his assumption that when Walt calls down from the rooftop, Walt must be trying to help people... these are all Aspie thought processes, and the lack of visible emotion -- the "flat affect"" as shrinks call it -- is typical of some subtypes of AS.
I have an Aspie friend whose personality is identical to Jay's, except that Jay didn't talk about anime for an hour. Like Jay, my friend has resigned himself to doing a boring job on the fringes of show business because he doesn't have the boldness or social skills to claw his way up the ladder.
I think Jay is just an ordinary Aspie who got a chance to witness some extraordinary events. Such a calm personality would be necessary for the narrator of this movie, because any other personality type would be freaking out or trying to interfere with Walt's rampage.
The virus that erases people from history represents the sadness that underlies college life; the knowledge that the friends you're having fun with now will eventually go their seaparate ways and disappear from your personal universe forever.
I need to watch this movie again, though. There is a lot of symbolism and I don't think we will ever know how much of it was deeply planned and how much was just thrown in for the adolescent joy of making the movie as weird as possible.
I wonder what the Jay character is up to now. Maybe he will have another adventure someday.