Basilisk Kotone:
You need to get through about half of the sky dungeon, if you check in on Kotone regularly you will see a Basilisk has entered the prison room as well. They will be right next to you, so do not move when you enter the room to check.
If you see the Basilisk, their back will be to you, and they will approach Kotone, let the scene play out.
Kotone Aelo: It is simple, lose the boss fight in the sky dungeon, I don't know if winning the mini-game is part of it but if you lose you get a mini-game animation.