Var not passed to a page question

[eluser]R_Nelson[/eluser]
my basic function for a page loos about like this

Code:

$data['title'] = 'Home Page';
        $data['discription'] = '';
        $data['keyword'] = '';    
        $data['main_content'] = 'site/index';        
        $data['query'] = $this->db->get('blog');
        $this->load->view('includes/template', $data); // calls template

how can i set say $data['username'] = '' without have to put it in the functions that are not used?

the reason im asking is when im on say the login page i dont want to have to load a blank left or right panel!