If the only unarmed attack the daredevil character has is a Fist, then the attack will be nonlethal. However, the Nonlethal trait says, "You can use a nonlethal weapon to make a lethal attack with a –2 circumstance penalty." The Fist Strike can be described as a kick, elbow jab, headbutt, etc., but it still uses the traits of the Fist.
However, some ancestries give other unarmed attacks. The Razortooth Goblin heritage gives a jaws attack that deals 1d6 piercing damage and has the finesse and unarmed traits. It is lethal. The Cactus Leshy heritage gives a spine attack that deals 1d6 piercing damage and has the finesse and unarmed traits. It is also lethal. Clawed Catfolk heritage gives a claw attack that deals 1d6 slashing damage and has the agile, finesse, and unarmed traits, and catfolk's Saber Teeth feat grants a jaws attack that deals 1d6 piercing damage and has the unarmed trait. Ironhoof Centaur heritage gives a hoof attack that deals 1d6 bludgeoning damage and has the finesse and unarmed traits. The Steelhoof feat increases the damage to 1d8. Etc.