Aion is an Eevee evolution that can only evolve though love. Love of its trainer, its friends, or even love to protect. As long as the love is strong enough to stand through ALL obstacles, Eevee can evolve into Aion. There's also one key difference between Aion and the other Eevee-eon-evolutions: if Aion for some reason loses love, or can't stand up to even ONE obstacle to their love for something or someone else, they can revert back into an Eevee or an Eevee sized Aion. Aion's size usually ranges from 2.5-4 ft tall although some cases have been reported of larger still. However, usually when an Aion is smaller than 2.5 ft, it appears to be going through a depressive state.
Power among Aion's is based on how loved they feel and how much love they give out. Therefore it's almost impossible to tell who will win in a battle between two equally leveled Aion. Also, just because one Aion beat the other, doesn't mean that the losing Aion doesn't love enough OR get enough love. Since love is measured differently between different entities, it could just be the personalities of the Aions that determined the win-loss.
Aion's appearence is relative to their trainer and how they percieve the world and love itself. Therefore, any particular Aion's appearence may change over time as they experience more of life and the world. However, all Aions' do seem to have a basic body-style. IE: Because Aion is an evolution of Eevee, Aion shares the standard facial-style as they other evolutions that remind many of the face of a fox.