Boros Challenger is a uncommon Creature — Human Soldier card from The List (PLST), collector number GRN-156. Current market price: $0.34, 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.