So, if you're looking for a unique Roblox username, then consider one of these:
Set your display name
Roblox now allows all users worldwide to change your Roblox account's display name in the account settings as a free feature. This is different than Changing a Username. How do I change the display name? Log into your account (If you can't log in, try these steps for resetting your password.) Go to account Settings
On Roblox, you can change your account's username in the account settings. It costs Robux to make this change. However your Display Name is a free option to change how your name appears in an experience or game.
Log into your account (If you can't log in, try these steps for recovering your password .)
A list of previous usernames can be found on the Account Settings and Profile pages, along with People search. You can change back to a previous username, using the above method, which also costs Robux.