Itami no Umi

Yuya Azuma has been continuously bullied since he started school. However, he manages to hold on through it all. One day, his foster sister, Yuka, whom he secretly loves, attempts to kill herself.
Bullies Michiru Ota and Aina Shindo unexpectedly calm down after Yuka is hospitalized. Yuta feels the change in their attitudes, but he doesn't know why until Yuka regains consciousness in the hospital. She doesn't recognize him, and she suddenly grabs his dick and starts sucking on it!
Yuya is terribly shocked at the sudden change in Yuka. He goes home and finds her diary in her room. Even though he feels guilty, he opens the diary and begins to read. He finds out the horrible truth: Yuka was gang raped in her efforts to help him!!
Yuya swears revenge on the bullies and his teacher who didn't try to help...

Our hero swears revenge for his foster sister, who has become mentally deranged because of the gang rape.
Here's a list of his targets:
Michiru Ota: Leader of the bullies and daughter of a politician.
Aina Shindo: Michiru's obedient follower.
Eri Miyamura: Holds a strong hatred towards Yuka.
Mariko Goto: Yuka's teacher.

Will Yuya be able to take out his revenge for the girl he loves?

[From Gamelex]