Donald Blake, Guise of Thor

The transformation is the whole conceit here, and the mechanic that carries it is unusually literal: a frail 1/3 with lifelink who, for a heavy activation, sheds the human doctor entirely and becomes a God Warrior Hero with two +1/+1 counters and flight. Power-up does something most creature-upgrade abilities never bother with: it rewrites the type line. He loses every other creature type when he powers up, which is a real cost in any deck built around tribal identity, and the one-shot restriction (each power-up activates only once) means the small early body isn't a launchpad you can spam, it's a threshold you cross once and cannot recross. The cost wrinkle is the part worth studying: activating him the turn he enters shaves the price by his own printed cost, so the fastest line rewards cashing him in immediately rather than protecting a 1/3 across multiple turns of open mana. That tension defines how he plays. Lifelink on the small body is nearly irrelevant; lifelink on a flying, counter-stacked God Warrior is a clock that pads your life total while it closes. The design is asking you to decide when the doctor stops being a liability and starts being a finisher, and the answer is almost always the turn he arrives, if you can afford it.