No, in PoR, Aether was a Occult skill that Ike could learn after his promotion (in RD, he gets it automatically after promoting). Criticals work completely different from skills, so Aether could activate even if you had 0% critical chance and the damage Aether does is calculated differently than critical hits. Specifically, Aether acts as two hits, the first one being normal damage while also healing Ike for the damage he dealt (shame that wasn't replicated in Brawl
) and the second hit ignores the enemy defense stat.
</nerdy correction>