Players that would like to, can setup a shared inventory, separate from a guild storage and different than marriage.
In marriage, your inventories become one, they can see all your stats and inventory items, no matter what. In this, it is a tab that is in addition to your normal inventory. This tab was labeled Sachi.
From a translation of the Sword Art Online light novel:
Charging into my hotel room, I immediately opened the storage box installed in the room, taking from the item window that popped up all the restoration, detoxification crystals and potions and the like. Although these alone could count up a hefty balance sheet, I would not pity it even if all of them were used up.
As soon as I took out a single-handed sword from my collection, confirmed its durability, I took off the sword on my back that I used to fight the ants and exchanged it. Then I also exchanged my leather coat and armor and everything else for new items. When I had finished, I was about to close the window when I saw my inventory and stopped my hand.
There, in addition to the «Self» written there, my own inventory page, was another label that read the name «Sachi».
This is the result of a very good relationship between two players, but one which has not progressed to «Marriage» —such players set their own common items window. This is different from the way that all items in marriage are shared in that only items in this separate window are shared.
Sachi, who had never asked for a confession or to hold hands previously, requested shortly before her death to set up this window. When I asked for the reason, she gave an answer that was difficult to accept, that it was for easily exchanging healing potions and similar items—if this was the purpose, there was clearly already a guild open that could be used for that. But I nonetheless agreed, and set up this window to share only between Sachi and me.
Though Sachi died, this window still remained. Of course, the friend list would also still retain Sachi's name, but it would be a grey that could not be contacted. And the few remaining healing potions left in the shared inventory, these would also not be used. After half a year, even with the guild page deleted without feeling, I was unable to remove the label of Sachi's name. Of course—the reason was not that I believed she could be resurrected—I just couldn't forgive the self that would have been able to feel better after having deleted her name.
I only recovered and closed the window after looking at Sachi's name for ten minutes. It was two hours until midnight.
– Sword Art Online Volume 02: Aincrad, Chapter 4, Part 3