Kasumi Kishimoto is 15. Having arranged to stay with her aunt, she is surprised to learn that the other four people in the house are all male.
The manga was originally 5 volumes long. Then it was reprinted in bunko 3 volumes (each bunko volume has more pages than the original volumes), and after that, it was reprinted again in 2 wideban volumes (even more pages per volume). Basically, the content and the length of the story is the same, they just come in different formats.