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Having read the manga, all I can tell you is that Shiki takes quite awhile to get into things. It'll start off with people dying left and right, and Natsuno generally being a jerk.

He gets wayyy better later on, though. *hint hint*

If you REALLY want to know:

 MAJOR Spoiler

The "Kanemasa" (i.e. the people that moved into the castle) are actually vampires. Well, one of them isn't...but whatever. Tatsumi is their most elite henchman, and appears to be some sort of werewolf-vampire hybrid thus making him unaffected by sunlight. Only a fraction of the people who end up killed by the vampires return from the grave as vampires, or as Seishin (and later Sunako as well) calls them, Shiki (literally "Corpse Demon"). Megumi becomes one of them...but still acts as frivolous as ever (there's a chapter that portrays the Shiki in a humorous fashion, Megumi being one of the main contributors to the humour). Natsuno's friend (forgot his name...) falls victim to a jealous Megumi, and he then feeds on Natsuno later on. By this point, Natsuno is already shown to not be that much of a jerk and loner.

Shiki tends to get a little confusing because it skips between Natsuno & Co. and the doctor quite often. Try not to get confused. :x