As always, each Mega-Pack will contain 1 card of each foil rarity (so 1 Secret Rare, 1 Ultra Rare, and 1 Super Rare in each pack) and all cards included will be in their original booster set rarity
not only can you use him to pop an additional card with Gangaridai to get more bang for your buck, you can use him with Gustav Max to clear the field a little and get in some extra deck, and you can also use him with Dora in order to directly disrupt your opponent's plays during their turn, whether you need to actually protect Dora or not
Cinderella Ballroom Sensation is a Rare card, denoted by the bronze triangle at the base of the card
Typically you see the Demon alongside Deaths Shadow as extra copies of the Avatar which creates another win condition for the strategy
It also ties into the Knight part of the deck
Limit of 15 cards can include powerful hand traps like Nibiru, Dark Ruler, and Evenly Matched