If this card is Normal or Special Summoned: Place 1 Guard Counter on it. This card gains 300 ATK for each Guard Counter on it. Once per turn: You can target 1 other face-up card you control; remove 1 Guard Counter from this card, and if you do, place 1 Guard Counter on that target (if that card would ever be destroyed, remove 1 Guard Counter from it instead).
Card Guard would be a great addition to Toon decks since it can protect cards from destruction. Toon Kingdom is the most important card on Toon decks and Card Guard give it an added protection while having a beater on the field.
You can use Card Guard's effect to other copies of it. The attack of the copy will increase up to 2200 and will have the added protection from being destroyed. It can also swap their Guard Counters so both of them can have 1900 attack and protection from getting destroyed.
Card Guard is a nice support to Gemini Monsters since it can give them survivability and time to gain their effects. These 2 cards benefits the most since once they are removed from the field, the monsters that is summoned by their effect will be destroyed.
Because that part of Mirror Wall's is a Maintenance Cost. If you cannot/do not pay the cost, then the card WILL be destroyed and cannot be prevented from destruction by any card effect.