Aether

Aether (天空, Sky in the Japanese version) is a mastery skill learned by Ike in Fire Emblem: Path of Radiance and Fire Emblem: Radiant Dawn.

Path of Radiance
The Aether Skill first appeared in Path of Radiance. The skill can be taught to Ike directly following his promotion after Chapter 17 of Path of Radiance. To teach the Aether Skill to Ike you need to have an Occult Scroll handy after his promotion. Two such scrolls can be obtained before his promotion, and can thus be used right away to teach him the skill. If those scrolls have been used another two are obtainable later on in the game.

In Path of Radiance it is described as having two attacks. One that is the mastery skill Sol and one that is the mastery skill Luna. This means that the first attack will restore HP equal to the damage done to the enemy and then will half the enemy's defense to do even more massive damage. This is considered to be one of the best Mastery Skills in the game, because of it's ability to keep Ike's HP up and still damage an enemy character, but also because of it's high activation rate.

In Path Of Radiance, Aether's activation rate is equal to Ike's actual skill stat. What makes this particularly good is that Ike can easily max his Skill stat such at 27, giving him a 27% chance of using Aether per attack. This is quite good compared to other mastery skills due to the fact many Mastery Skill's activation rate is the character's skill stat divided by 2.

Aether is useful for defeating the Black Knight and to a much lesser extent Ashnard. The Luna part of Aether deals a lot of damage, almost half the Black Knight's HP if Ike has maxed Strength.

Radiant Dawn
Aether reappeared in Radiant Dawn, again as Ike's Mastery Skill. However in Radiant Dawn Ike will automatically receive his Aether skill upon his story-caused promotion to the Vanguard class. This is similar to the other character's in Radiant Dawn automatically receiving their possible Occult/Mastery skills upon reaching a 3rd-Tier class.

In Radiant Dawn, it still has the same effect, still combining the effects of Sol and Luna. However it is significantly less effective because of it's activation rate being cut in half. Meaning that instead of its activation chance percentage being equal to Ike's actual Skill Stat, the activation rate is Ike's skill stat divided by 2. This still is not a bad rate, considering he still has a 20% chance of using it, if he maxes out his skill stat.

Also in Radiant Dawn, all of the Mastery Skills have received a significant upgrade. All Mastery Skills deal increased damage upon hitting the enemy, along with their original effect. This is opposed to the Mastery Skills of Path of Radiance that only dealt extra damage when their effect specified so (like in Luna's case). This extra damage dealt means that more often then not the Mastery Skills will simply outright kill their targets. This fact diminishes Aether's appeal as an incredibly effective skill. This is offset by the fact that if Ike maxes out his Str. stat, he will always do 56 Dmg. to enemies, healing himself with the same amount.

Super Smash Bros Brawl
In Super Smash Bros. Brawl, it is used by Ike as his 'Up B' and so it works as his recovery move. It resembles the first hit of the actual Aether attack where he throws his sword into the the air, jumps up spins and lands. It is considered to be a less effective 'recovery move' when compared to other recovery moves in the game as after jumping upward Ike falls rapidly to the ground (sometimes into the bottomless pit). Also, in SSBB, Ike has his Ranger outfit, suggesting he is of that class but wields Ragnell and uses Aether, two things he could not do at the ranger class. This has caused some agitation among fans of both series.

Also, in Brawl, Ike's Final Smash is a more powerful version of Aether, named Great Aether: Ike waves his flaming sword at an enemy and makes a sweeping upward strike, sending an enemy (if it hits them) into the air, he then proceeds to strike the enemy several times doing a great deal of damage, before giving a last hit where he rapidly pushes them downwards to the ground with his sword. Once Ike lands, pushing his enemy into the ground, a huge explosion erupts from the impact area which blows the enemy off to either the left or right, often sending them offscreen. Also, any additional enemies who are in close vicinity of the explosion resulting from Ike's downward strike at the end of Great Aether will also be damaged and tosses to the side.

Trivia

 * Aether may be based on a Mercenary's critical attack, which makes Ike seem more akin to the Mercenary or Hero classes.
 * Ike also jumps a lot lower in Radiant Dawn than he did in Path of Radiance, possibly due to his extra bulk in the sequel.

Etymology
In Greek mythology, 'Aether' refers to the pure, pristine air which the gods breathe, at a higher level than earthly air ('aer'). Aether is personified as one of the earliest primordial Greek gods.