Adult PC Game: Earth's Last Guardian v0.2.0d by EcchiYoYo Productions Win/Mac Archive

You play as an average guy, living with your sister after a worldwide pandemic wipes out 96 % of the world’s population. After your uncle dies a mysterious letter appears in your room, as soon as you finish reading the letter a strange door materializes in your room, drawn to the door you cannot help but to open it and step through. And so, starts your adventure, you will meet many new friends and a great many new enemies.

How you play the game is mostly left up to you, you can play it out on earth ignoring the strange dimension through the doorway or spend your time in their and finding out the legacy of your family and the guardian that came before you.

A small hint to progress past the town being the only thing unlocked:

Earth's Last Guardian v0.2.0d by EcchiYoYo Productions Win/Mac Gameplay Preview