Fire Urchin

The 1/3 body is the tell: a spell-deck blocker built to survive the small creatures it would trade down to, while its power waits on the +1/+0 triggers to make attacking worthwhile. That growth is the whole engine, but its shape is more modest than it reads, because the buff lasts only until end of turn. This is not a creature that accumulates spell-casts into a permanent clock; each turn resets it to a 1/3, and the reach it gains is spent within the same combat step you cast the spells that built it. So the payoff is bursty rather than incremental: on a turn where you fire off two or three cheap instants and sorceries, the Urchin swings as a real threat, and the trample means the pumped power spills past a chump blocker instead of being absorbed. On a turn where you hold your cards, it goes back to blocking. The trigger is also narrower than "noncreature spell" implies: it fires on instants and sorceries specifically, so artifacts and enchantments feed it nothing. The structural cousin is prowess, and the instructive difference is which stat moves. Prowess adds toughness too, buying survivability through the turn; the Urchin only grows in power, so it never gets harder to kill, just harder to block profitably. It rewards a spell-dense shell that wants a single attacker to punch through while the burn clears the rest.