Gisela, Blade of Goldnight
A damage doubler and a halving shield rolled into a single seven-mana body, and the math is what makes the angel terrifying rather than merely big. The doubling clause turns every attacker, every burn spell, every combat blocker you control into a force that hits twice as hard against an opponent or their permanents; the halving clause rounds up in your favor, so the same swing that erases their board barely scratches yours. The asymmetry is total: she warps the damage rules in one direction only, which means a five-power flier with first strike is really a ten-damage threat that closes a game in three connections, and a Lightning Bolt aimed across the table becomes six. The first-strike keyword is the quiet half of the package, since combat damage that lands before a blocker can swing back is already doubled by the time the creature would have struck. What balances all this is the price and the fragility: she cannot attack the turn she lands, costs enough that you have committed the board to her, and dies to the same removal she does not protect. But left alive for a turn cycle, she rewrites the calculus of every source of damage on the battlefield, friendly and hostile alike, and that one-directional rule-bending is the rare design that scales with everything else you are already doing.
