[ST04-010_p2] - Who's.Who (One Piece Promotion Cards: Tournament Pack Vol. 3, Participant)

Can I use this Character card's [Trigger] effect to play this card if I have no DON!! cards that I can return to my DON!! deck?

Yes, you can. In that case, because you don't have enough DON!! cards, you cannot activate the [On Play] effect.