Private Key is basically the password to access your account. In the Blockchain world, you only need a Private Key to access an account, without a need to know other things like the username, or anything at all. Private Key is all you need.
This simply means who has the Private Key will be the owner. You cannot modify your Private Key in any way, as it’s provided by the Blockchain. In case your Private Key is leaked, you have to move everything to another account immediately. The reason is your account is not secured anymore.