She costs four in Boros, with double strike and a two power so that's four she's dealing base and could increase massively from there
Linvala, the Preserver: more like Linvala, the Battle Cruiser, a slower version the previous Linvala (2WW 3/4 Flying "Activated abilities of creatures your opponents control can't be activated.") These effects are good, but a six drop that does not on its own wins games is a Standard-only card
The series became a smash hit and would go on to sell more than 40 million copies
"Lunalight Fusion" or "Heavy Polymerization" can use materials from the Extra Deck under the right situations (incidentally, both require the opponent to control a monster while the former requires that opponent's monster to be one that is Special Summoned from the Extra Deck) while "Wolf" and "Masquerade" can banish materials from the GY for their Fusion Summon effects
It needs your deck to provide the effects worth doubling, and at five mana it rewards decks that can protect it or benefit from it quickly