Prepend to start of WordPress nav loop

Instead of using a Walker in this instance you can use the items_wrap parameter of wp_nav_menu.

$args = array(
    'container' => 'nav',
    'container_class' => 'c-nav c-nav--vertical c-nav--collapsible',
    'menu_class' => 'c-nav__collapser',
    'theme_location' => 'secondary',
    'items_wrap' => menu_wrapper()
);

wp_nav_menu($args);

In the called function you can use the arguments for %1$s (id), %2$s (class), and %3$s (all the list items).

function menu_wrapper() {
    $wrap = '<input type="checkbox" id="responsive-nav-toggle" />'
    . '<label class="c-nav__collapsible-label" for="responsive-nav-toggle">'
    . 'Explore this section… <i class="c-icon c-icon-down-open"></i>'
    . '</label>'
    . '<ul id="%1$s" class="%2$s">'
    . '%3$s'
    . '</ul>';

    return $wrap;
}