Will to Fight is a Determination deck specific effect card. When used, it adds +4/+4 to the selected monster and then puts it back in the deck. This will act like 'resting' the monster recovering any lost HP and removing any additional affects and then adding the +4/+4 on top of that. Multiple can be stacked onto the same card to give it a far higher attack. Best used with Flowey, or later in game when less cards are left in your deck, giving a higher chance of drawing a card with Will to Fight applied to it.