You can add a card to your hand with “Guardian” in its name as long as it’s not listed on “Arsenal Summoner”, such as “Guardian of the Throne Room” or “Guardian Sphinx”.
You must show your opponent the card you retrieved with “Arsenal Summoner”.
If you don’t have any “Guardian” monsters you can retrieve when you activate “Arsenal Summoner”’s effect, you must show your Deck to your opponent to prove it.