Aether is not a critical. Aether is an ability that relies on Ike's skill in relevance to the chance of it activating on an attack. Aether is composed of two attacks.
Sol, the jump and the landing, which in the game steals HP.
Then Luna, the second hit where he lifts up the sword and hops backwards. This ignored defense.
Ike has two different critical animations, three counting his Lord-self. The first is a simple leap slash from a distance. The second is the typical sliding slash through the opponent (probably his dash attack). The last is a finishing swing where he swings with enough momentum to turn him around and he faces the other way as his foe falls to the ground in defeat.
Edit - I forgot to mention the fourth, the critical animation for ranged sword magic. Just spins it around before using it.
