Sierra, Nuka's Biggest Fan is a rare Legendary Creature — Human Citizen card from Fallout (PIP), collector number 25. Current market price: $0.37 (foil $0.71), TCGplayer market prices via Scryfall.
The Nuka-Cola Challenge — Whenever one or more creatures you control deal combat damage to a player, put a quest counter on Sierra and create a Food token. Whenever you sacrifice a Food, target creature you control gets +X/+X until end of turn, where X is the number of quest counters on Sierra.