Vault

From HammerWiki

Each character has a vault accessible by talking to a Banker. As you would expect, the bankers can be found a Bank, but they are also spread-out in strategic locations in the capital cities. Moving items to or from the Backpack or the in-game Mail system is a simple drag and drop affair.

Each vault is unique to a specific character, so passing items to other characters (on the same account or on different accounts) is best done via Mail.