You must obtain all the following passive character upgrades. You can check your progress at the pause menu.

Reload (level 6)
Bullet Magnet (level 3)
Coin Magnet (Level 3)
Carry Item (Level 3)
Life Insurance (50%)
Discount (40%)
Backpack
Poison Immunity

All of them except Poison Immunity can be obtained at a vending machine. In order to get Poison Immunity, find a vial of Poison Serum in a hidden room. If you do not have all the upgrades except the Poison Immunity, consider saving the Vial of Serum in your bank until you have one, due to its rarity. To locate hidden rooms, begin the game and hug the left-side wall. While always facing the wall, so you can see when the pointer changes to "use" when you find a hidden wall or pass by a door. Do not open it until you have "scanned" all the walls in the current room. Repeat the process until you either get a Hacked Sensor (beeps when hidden rooms are nearby) or the serum itself. Also consider saving the Hacked Sensor for finding the Ritual Blade, High Heels, and Boom Box, because it speeds up the process of finding any of the hidden items.