On Nostr you are your npub you are not your username.
There is also your nip-05 that identifies you in case someone else wants to impersonate you.
For example my nip-05 is :
isolabellart@nsec.app 
And no one else can have it like it because it's tied to my npub. 

 Ohh. How can I get one of those. 

 https://nsec.app/ 

 Thanks