Key items:

These are necessary to finish the game.

Glass Jar Jar of Liquid Key

Special items:

These open hidden rooms on the second floor of Building B.

Sheets of Numbers Mouldy Bread Old Cigarettes Blueprints
Букварь Конфета New Cigarettes

Standard items:

These do absolutely nothing!

Faceless Matryoshki Porcelain Figure Picture of You Fermented Soda
Sparrow's Skeleton Icon Old Batteries Tarragon Soda
Precious Cup Worn Penny Burnt Poem Old Cola
Canned "Meat" Cat Cross-Stitch Paints Steel Bearings
Onion-in-progress Honey Cake Garlic Someone's Letter

Ending Guide

There are two endings, and both diverge from the final room with the pit.

Happy End Simply walk towards the ladder in the far left corner of the room, or choose "No" either time when prompted by Nika.
True End Choose the following choices in the conversation with Nika: Yes -> Either Option -> Yes

password: fancyrat