Keleth, Sunmane Familiar

Delegation dressed as a creature. The Horse itself does almost nothing (a fragile attacker that grows only when it swings), and its ability is dead until it resolves and starts watching combats. From that point on, any commander you control that attacks fattens up permanently, which is where the real function shows: pair it with an aggressive partner legend and that partner's commander-damage clock ticks down faster and keeps compounding wherever the board grants extra swings. Because this is a commander in its own right, its attacks feed its own trigger, so it can bulk up alongside the partner rather than sitting purely as an anchor, though the counters are worth far more funneled onto a single dominant swinger built to close a game. Partner is doing enabling work here, not payoff work: the payoff scales best once you have a second legend to run beside it, so the card behaves less like a threat than like a two-mana permission slip to field two commanders where one carries the beatdown. It sits among white's long tradition of support pieces that ask you to build around someone else, scanning as filler until you notice the job is scheduling a lethal commander-damage clock and then accelerating it every combat.