[OP10-069] - Fighting Fish (Royal Blood)

I attacked with this Character while it was given 1 DON!! card, but that DON!! card was removed when using the DON!! −1 part of this card's [When Attacking] effect, causing the number of DON!! cards given to this Character to become 0. Can I still K.O. 1 of my opponent's Characters with a cost of 1 or less?

No, you cannot.