For those who care, I found out and/or determined the names of the characters who are provided with sprites.
Spoiler for Images 1:
Top row (left to right): Michiru Sakurada, Midori Kusunoki, Yukari Sakuki
Bottom row (left to right): Yoshihito Kanamori, the Headmaster/Principal, the "Black Tea Gentleman" (?)
Spoiler for Images 2:
Top row (left to right): Higanbana, Takeshi Nonomiya, Sumire, Marie Moriya
Bottom row (left to right): Hamelin/Hameln, Public Health Teacher/Nurse, Hikaru Nihei
Oh, and there are two other characters whose sprites were never released prior to the game's release:
I also managed to extract the sprites of the game. However, unlike Higurashi and Umineko's sprites, each sprite is separated into two files: one for their heads and expressions, and the other for their stationary bodies.