But why Fortisphere and no Pawpsicle
The Sapphire cards in this deck are all very Hero focused and offer a few ways to manipulate your deck (drawing a card, putting a card facedown in your inkwell, looking at the top 3 cards of your deck and re-arranging)
This color combo does well by going aggro and aggressively questing with high-lore characters
However, Kuja attempts to steal it with the Invincible, but is thwarted by the evil Garland (different from the one in Final Fantasy I )
Pairing him with someone like Namaari, Morning Mist could prolong his time on the field thanks to her Bodyguard ability while also granting him an extra lore
So, even if all you wanted to do was have a few of the Robin's Pal variants in your deck, they could all be played at the same time or one right after the other, making a wall of defense for your other characters