Naisho no Tsubomi

Each volume depicts a different heroine.
Volume 1: Main character of the story is Tachibana Tsubomi (立花つぼみ), a fifth grader dealing with issues such as her mother's pregnancy, her first period, and the strange feelings she has started to get when she has been around boys.
Volume 2: Main character of the story is Kusakabe Tsubomi (草壁つぼみ), a fifth grader, who encounters an alien wanting to do research of earth. It turns out he has picked Tsubomi to be his research subject.
Volume 3: Shiina Tsubomi (椎名 つぼみ)
Volume 4: Katanashi Tsubomi (小鳥遊 つぼみ)
Volume 5: Aizaki Tsubomi (相崎 つぼみ)
Volume 6: Nike Tsubomi (仁家 つぼみ)
Volume 7: Yagi Tsubomi (八木 つぼみ)
Volume 8: Honda Tsubomi (本田 つぼみ)

Note: Won the Shogakukan Manga Award for children's manga in 2009.