You have to dive into comment template in your theme and find out the line where username is printed. Then use the following function in functions.php:

function alter_username( $username ){
    return substr( $username, 0, 1 ). str_repeat( '*', strlen( $username ) - 2 ) . substr( $username, strlen( $username ) - 1 ,1 );
}

Then use the function in comment template or wherever you want to show altered username like:

echo alter_username( $username );