Futakoi

For Aizawa Yuuichi, returning to the town where he spent his childhood has been an unusual affair. Yuuichi, it seems, has forgotten his memories of the past, including encounters with several young ladies who would love nothing more than to see him again. As the days pass, flickers of the past began to surface, revealing more and more of the dark events that once came to be. As the reuniting continues, Yuuichi soon starts to realize that his past may be far more complex than he once believed it to be...

Wakura is so confident of getting into his chosen high school, he takes no other tests. Unfortunately, he gets every question on his entrance exam WRONG. He then finds out that he was entered by recommendation into a high school on an island. Once there, he finds out that he has 12 (or perhaps 13?) adopted sisters. The story revolves around his growth as he interreacts with the 13 sisters.