Heart of the Underdog is a Continuous Spell Card.
Spell Cards are usually activated from the hand on your turn unless their type gives another timing window.
On the current TCG and OCG Forbidden & Limited lists tracked here, Heart of the Underdog is Unlimited.
YGOPRODeck lists Heart of the Underdog for these formats: GOAT, OCG GOAT, Duel Links, Common Charity, Edison, TCG, OCG, Master Duel.
A listed print of Heart of the Underdog appears in Dark Revelation Volume 2 (DR2-EN032) at Common rarity.
Among markets shown on this page, the lowest listed snapshot for Heart of the Underdog is about $0.10 on Cardmarket (prices change frequently).
