The MV-BANK is a device that stays in Ace's wallet. It usually takes the form of a card (similar to a bank card) but can change form if needed. It can be used in any kind of ATM and online purchase in any world. Shaking the card will cause physical coinage to fall out. It struggles to create bank notes however.
Obtaining an MV-BANK
Any multiworld traveller can have a MV-BANK. Obtaining one not a difficult process. MV (the company that produces the Multiverse items) tracks people as they travel through multiple worlds, having money stored in three or more worlds and stating your desire to have access to the money while in a fourth triggers a representative to leave the MV-BANK in your base of operations. It comes with a letter from MV with a list of other items they offer and how to buy them.