It's so popular because it only costs 3 ink to play, and its two abilities allow you to both gain lore and draw cards, two things that help you win the game
Under ordinary circumstances, Belle & Beast Certain as the Sun will come down and become an immediate target
The lack of red mana in this theoretical deck puts a limit on the number of Final Fantasy Wizards you can consider, but theres still Venat, Heart of Hydaelyn, who lets you draw a card every time you cast a Legendary card
When he transforms into Ifrit, all Ifrit's abilities are about fighting other creatures, because in the game, you're always fighting those big epic boss battles as Ifrit
Clean up after yourself, mind your language given that the game caters to children as well as adults, and just generally be a good citizen