Sorry, but does anyone know what the key items section in the game is for?
You can buy engagement rings from the jewelry in the town or when you are immobile and have enough affection with one of the girls,you can buy one online if you dont have one in your inventory in My things section in the Key items one.
Key items tab from my things from inventory is with important things like engagement rings which make your partner into your wife and i dont know if the bug that made it so that you can marry one wife per town.
Hope it helps.
Also,i dont know if there are more items that are going into that category.