Boros Challenger is a uncommon Creature — Human Soldier card from Guilds of Ravnica (GRN), collector number 156. Current market price: $0.09 (foil $0.36), TCGplayer market prices via Scryfall.
Mentor (Whenever this creature attacks, put a +1/+1 counter on target attacking creature with lesser power.) {2}{R}{W}: This creature gets +1/+1 until end of turn.