Helmut Zemo, Mastermind
The recursion here has a growth clause bolted onto it, and the two halves feed each other in a way most graveyard-spell engines never bother to close. On the declare-attackers step, you may recast a cheap flashback-style spell out of the yard, but only one whose mana value sits at or below Zemo's power; whatever you cast exiles after resolution rather than returning, and Zemo grows by one. That growth is the whole loop. Because it lands inside the same declare-attackers window, each attack lifts the ceiling for the next before blockers are ever declared. Open with the cantrips and one-mana burn, and the ladder climbs toward your heavier payoffs swing by swing. The exile clause is what stops this from becoming a true engine: every card you pull is spent for good, so it plays as a self-consuming toolbox rather than a repeatable loop.
The timing has a sharp edge. The recast fires inside the attack trigger, so even though it is optional, a retrieved counterspell only earns its trip if there is already a legal target sitting on the stack; you are answering what is in front of you, not holding an answer for later. The 2/2 body keeps the early triggers modest, which points the whole package toward a spell-dense build with a low curve at the bottom and the expensive answers waiting at the top, unlocked one attack at a time.
