Can you pay deflect cost with Kai'sa legend ability when casting a spell?

Ruling: Yes, Kai'sa's legend ability can pay for deflect costs when you cast a spell that targets, because deflect becomes an additional cost of the spell itself.

Nuances:

• Kai'sa's legend ability only works for spells, not abilities. You cannot use it to pay deflect for triggered or activated abilities.
• Reflexive triggers (like those from Falling Star or Icathian Rain) are an exception. Even though these spells create triggers that target, the triggers themselves do the targeting, not the spell, so Kai'sa's legend cannot pay for deflect in these cases.
• Targeting does not source up to the spell, unlike damage and killing effects which do source up.