Short Answer: The Holo fuel app functions as your Holo fuel account (‘wallet’).
Additional Information: Once Holo launches you will have your own identity, where your transaction history and Holo fuel balance will be stored. This works by using an immutable source chain that your transactions in Holo fuel are recorded to. This functions as your account. The sum of your inbound and outbound transactions in your transaction history determines your current balance.
It’s possible that, one day, a hardware wallet would support Holo fuel.