Keiichi - Keiichi is the GAR awesome charismatic speech-giving hero. I've always lied Keiichi. He learns from his mistakes between worlds, and his true colors are shown in Kai when he cares so much for his friends that
Mion - Mion has always trusted her friends. She is loyal and caring, and always tries her best for her friends. She's an absolute role-model. <3
Rika - Rika, Rika, Rika.. She struggles so hard, ni-pah's, and is in general a very well thought-out character. She fights almost endlessly just to live happily with her friends.
Hanyuu - She is so cute, and she really grows when she decides to become a part of everything.
Shion - Shion has really changed a lot. She has become trusting of her friends, and even looked after Satoko. She's had a lot of development.
Rena - I've always liked Rena. She is a very complex character, and while she doesn't do too much in Kai, I've always enjoyed her intelligence, intuition, love for her friends, and let's not forget, her psychotic side.
Satoko - Satoko has really matured. I could see this when she called Keiichi her "ni ni" in S1 and called Shion her "ne ne" in Kai. She finally breaks free of her uncle's abuses, which was a big step for her character.
Overall, Higurashi has many amazing, fleshed out characters. I love so many of them. <3