Kate Stewart is a rare Legendary Creature — Human Scientist card from Doctor Who (WHO), collector number 422. Current market price: $0.38 (foil $0.34), TCGplayer market prices via Scryfall.
Whenever you put one or more time counters on a permanent you control, create a 1/1 white Soldier creature token. Whenever Kate Stewart attacks, you may pay {8}. If you do, attacking creatures get +X/+X until end of turn, where X is the number of time counters among permanents you control.