Rena - Still my ultimate favorite in Higurashi, despite her lack of screentime in Kai, compared to other characters. Intelligent and adorable, plus a mean hatchet wielder to boot.
Keiichi - Of course, the hero of Higurashi with no doubt the most interesting of personalities among the characters, IMO.
Shion - Not much screen time, but her loyalty to Satoko was just honorable and lovely.
Akasaka - Honorable and kick-ass. What more can you wish for?
Irie - His involvement with the Hinamizawa project was definitely one of the more interesting aspects of the series. I can definitely say Irie found an official fan.