Heartfire Hero is a uncommon Creature — Mouse Soldier card from Bloomburrow (BLB), collector number 138. Current market price: $0.30 (foil $0.37), TCGplayer market prices via Scryfall.
Valiant — Whenever this creature becomes the target of a spell or ability you control for the first time each turn, put a +1/+1 counter on it. When this creature dies, it deals damage equal to its power to each opponent.