inserting custom li class to wp_list_pages

wp_list_pages allows you to include a walker function. That would give you complete freedom to add any classes you want, but it’s no a beginner’s method. You could start by reading this.

If you want a simple search and replace on the class, as your question seems to indicate, you can use the wp_list_pages filter like this:

add_filter ('wp_list_pages','wpse241119_replace_class',10,3);
function wpse241119_replace_class ($output, $r, $pages) {
  $output = str_replace ('page-item number', 'hvr-underline', $output);
  return $output;
  }