Gideon's Avenger

Most growth-counter creatures reward you for your own development: attacking, casting spells, sacrificing things. This one inverts the incentive entirely, keying off the opponent's tempo instead. Every time a creature they control becomes tapped, the Avenger swells, which means the opponent feeds it simply by playing the game: swinging into you, tapping a mana dork for ramp, paying a tap cost to activate a creature, even sending a creature-land into the red zone once it is in motion. The wrinkle is that the trigger fires on tapping, not attacking, so it punishes any deck whose plan leans on tapped creatures rather than only the aggressive ones. The body is the catch: a 2/2 with no evasion and no protection means the counters pile onto something the opponent can answer at almost any point along the curve, so the card asks you to defend a fragile clock long enough for the asymmetry to compound. Note where it goes quiet, too: a true board stall, where their creatures sit back and never tap to do anything, is exactly the situation that strands it, because untapped blockers and idle attackers never trigger it. It wants an opponent forced to keep tapping, which is why a controlling white shell suits it best, leaning on the opponent's own activity to do the work while small blockers buy the turns. Left feeding for even a few combat steps, it stops being a soldier and starts being a finisher.