How to use page metadata while using wp_list_pages()?

You can do it easily with jQuery by using .eq() to add a specific class to each i element.

Example:

Add a class to your i element from PHP.

<?php wp_list_pages('link_before=<i class="retina_icon"></i>&link_after=<br >'); ?> 

Then add another class to it with jQuery.

jQuery('.retina-icon:eq(0)').addClass("retina-icon-1");
jQuery('retina-icon:eq(1)').addClass("retina-icon-2");