Can I cast an action speed spell in response when my opponent casts Void Seeker on my Kai'sa that's on the battlefield?

Ruling: No, you cannot cast an action speed spell in response. You can only play actions while the chain is empty, and on other players' turns, you don't have the option of playing actions outside of showdown.

Nuances:

• You can never play an action "in response" - actions can only be played while the chain is empty
• On your own turn, you could play actions while the chain is empty, but not on opponent's turns unless during showdown