Can Kai'Sa legend ability be used to pay deflect costs?

Ruling: Yes, Kai'Sa legend ability can be used to pay deflect costs. Deflect cost is a mandatory additional cost that becomes part of the spell's total cost, and Kai'Sa can pay any part of a spell's cost whether it's the printed cost or an additional cost.

Nuances:

• This applies as long as the card being played is a spell
• Example: A Kai'Sa player can use the legend ability to pay the deflect cost when targeting a mighty Fiora with Stupify