Aurora Shifter

Energy is a resource most cards mint for someone else to spend; this Shapeshifter is the rare one that mints its own fuel and burns it entirely on self-improvement. The 1/3 body is a modest combatant that has to connect: combat damage to a player is what banks energy, one point for one on an unassisted swing. That arithmetic is the honest brake on the loop, because two clean hits, or a single buffed one, are what it takes to accumulate the the copy ability wants. Pay that cost as combat opens and the creature becomes a copy of a stronger body you control, carrying forward both the copy ability and the damage-to-energy trigger. That inheritance clause is where the engine compounds: once it clones your best threat, its subsequent hits generate energy at that threat's power, funding richer transformations than the original body ever could. Crucially, the copy has no duration: it stays locked in the new form indefinitely, so paying again on a later turn is a deliberate choice to overwrite a good shape for a better one, not an upkeep tax to hold the one you have. That makes each transformation a ratchet rather than a lease, and it means the copy ability only matters when your board has since produced something worth becoming. Point it at a bigger threat and the energy accrues faster; leave it be and it simply keeps swinging as whatever it last chose to be.