Agatha of the Vile Cauldron

The whole design turns on a self-referential loop that a 1/1 body deliberately refuses to close on its own. The cost reduction reads off her power, so at power 1 she is already shaving a full mana from every activated ability on the creatures you control, which is nothing to sneeze at: a static one-mana discount across a wide table is a real engine before you have done any work. But the reduction only bites generic mana, and the floor keeps it from ever zeroing a cost, so the reward scales with two things at once. Pump her power and the discount climbs; fill the board with dork-tap, sacrifice, and repeatable activated toolboxes and the discount multiplies across activations. The one-mana floor is the leash on the whole thing: she compresses costs rather than eliminating them, which means the density of activators, not the raw size of her power, decides whether the reduction is a convenience or a stranglehold. Her overrun ability is a creature activation too, so it feels her own discount and never actually costs its printed six: at base power it resolves for five, and less as she grows, which quietly rewards buffing her for reasons beyond the static effect. What makes her a genuine deckbuilding puzzle is that her power doubles as an input variable, feeding both the static engine and her own activation. A red-green commander whose power is less a stat line than a dial is an odd thing to hand a builder, and it is why she plays more like a cost-reduction widget with legs than a creature.