Hero Smiley is a special developed by Kleya Smith, created to remind herself to do the right thing. In a match, it attacks (or boosts) happiness, causing the enemy to become extremely happy and unable to fight.
It is classified as a random special, triggered by some random effect. Kleya has hacked this trigger to prevent her from hacking. Or, in clearer words, it only is triggered when she attempts a hack. The music notes "block" the hacking attempt, then a giant smiley face falls from the sky and hits the opponent. They then become extraordinarily happy, and pass out.