Defensively, Kay does a good job pressuring and draining time off the shot click initially. Where he's been beaten in half-court sets is the period after the point guard makes the first pass. Because he gets right up in a guy's jersey, players can make a move by him after catching the ball. It's mostly an awareness issue, not an effort or agility one, which is good.
Other potential issues is getting caught on screens, playing solid off the ball defense, and not getting switched onto a bigger player. I feel like he can improve on these aspects.
Overall, I'm liking his play and potential.