Mai, Jaded Edge

That 3-toughness back is the first tell that this is not a prowess creature built to swing early. Most prowess bodies want low toughness and high power: something you protect, pump, and race with. This one asks to sit. A 1/3 absorbs the early attacks a spell-dense hand tends to invite, staying alive while you develop a turn where casting matters more than attacking. The exhaust ability is why the toughness earns its keep. Sink three mana, once, for a double strike counter, and every prowess boost you get that turn suddenly connects for double. The math is smaller than it looks but the leverage is real: hold up interaction, chain two or three noncreature spells on the attack turn, and a 1/3 becomes a 3/5 or 4/6 with double strike, a lethal swing assembled out of a body that spent three turns doing nothing threatening. The once-only clause on exhaust is what keeps this from being an engine; it is a single investment that permanently upgrades your combat, but not a repeatable value loop. That is a slower, more deliberate read on prowess than red usually offers. Prowess normally rewards curving out and pressuring early; here the reward is deferred, contingent on you having the spells in hand and the mana to spare on the exhaust, and the payoff spikes only because the double strike counter doubles a boost that was always temporary to begin with.