Coral Trickster

The morph payoff here is small but precise: when the face-down 2/2 turns face up, you tap or untap a single permanent, and the whole card is built around choosing the instant to do it. Tap a would-be blocker before blockers are declared and the body resolves into a 2/1 that swings unimpeded; untap your own permanent for a surprise blocker or to re-fire a tapped ability; tap a land during an opponent's turn to shave a mana off a key spell. The flip cost is cheap enough that the effect costs almost nothing once you have committed the to play it face down, but the trigger fires once: morph is a one-way action, so this is a single well-aimed tempo nudge, not an engine. The bluff carries as much weight as the effect. The face-down 2/2 could be anything, so an opponent has to respect a possible tap during a combat or counterspell window even on turns you hold the mana and never flip it; the threat of the flip is live every turn the card sits unrevealed, even though the flip itself is not. It is a low-ceiling design by intent: a utility Merfolk Rogue whose value is counted in one permanent tapped or untapped at the right moment, propped up by morph's information game, rather than in any board swing.