Okay... because I'm really depressed to get all of the endings, I found this website. It's a complete walkthrough of Hakuisei Ren'ai Shoukougun.
Finaly I can play and get Hatsumi's ending. It took me forever before I found this website. When I saw the date, he/she posted this walkhtrough around 2 years ago! I can't believe I believe it took me 2 years to find this game walkthrough.
Here's the website : http://brightbell.main.jp/kipima/game/Sirokoi/
Oh and.. I've already tested it so don't worry about making a mistakes ^^;