Adult PC Game: EcchiYoYo Productions - Earth's Last Guardian Ver.0.1.3 Win/Mac + Patch 0.1.3b Archive

Overview:
The game takes place in a post-apocalyptic Earth that has been ravaged by a viral outbreak, this outbreak reduced the world’s population by 96 %.
Our protagonist is an average guy living a mundane life with his sister, go to work eat and, sleep, rinse and repeat day after day.
This all changes the day a strange letter appears in his bedroom informing him of his uncle’s death, and that he must now take up his family’s mantle and join an eternal war.
The game is mostly a sandbox, you chose what you want to do and who you want to build relations with.
A small hint to progress past the town being the only thing unlocked: