Can Defy counter Yasuo, Remorseful's triggered ability?

Ruling: No, Defy cannot counter Yasuo's triggered ability. Defy can only counter spells, and Yasuo's trigger is an ability, not a spell.

Nuances:

• Defy specifically says "counter a spell" and not "counter a spell or ability"
• Cards like Not So Fast have different text that would allow them to counter abilities