You are looking at the HTML representation of the XML format.
HTML is good for debugging, but is unsuitable for application use.
Specify the format parameter to change the output format.
To see the non HTML representation of the XML format, set format=xml.
See the complete documentation, or API help for more information. 
<?xml version="1.0"?>
<api>
  <query-continue>
    <alltransclusions gatcontinue="Character_Box|566" />
  </query-continue>
  <query>
    <pages>
      <page pageid="15" ns="0" title="Issei Hyoudou" />
      <page pageid="42" ns="0" title="Rias Gremory" />
      <page pageid="75" ns="0" title="Akeno Himejima" />
      <page pageid="79" ns="0" title="Asia Argento" />
      <page pageid="93" ns="0" title="Ddraig" />
      <page pageid="111" ns="0" title="Sirzechs Lucyfer" />
      <page pageid="204" ns="0" title="Yuuto Kiba" />
      <page pageid="222" ns="0" title="Koneko Toujou" />
      <page pageid="230" ns="0" title="Vali Lucyfer" />
      <page pageid="558" ns="0" title="Xenovia Quarta" />
    </pages>
  </query>
</api>