The first kanji on four girls' last name represents direction. Tou in Toujo Aya's name has the same kanji as east, Nishi in Nishino Tsukasa's name means west, Kita in Kitaooji Satsuki means north and Minami in Minamito Yui means south. Also, Naka in Manaka Junpei's name means "center" or "inside". This can be used as a diagram as which girl should Junpei choose or literally which direction would he choose.
Taken from Anime News Network