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Pilot of the G-Attacker.
Essentially the main protagonist of the series, Eiji is first show as a woman... or rather, disguised as one. The disguise is so good in fact that he is hit on by several men at Sandman's ball. He quickly takes his leave though when Sandman begins speaking, slipping through the castle while attempting to locate his sister, Aiaka. Of course, his infiltration does not go off without a hitch. While evading a patrol he crashes through the door into the maid's changing room, somthing he catches loads of flak for later (mostly from Mizuki).
Eiji works his way down into the hanger where the Grand Kaiser is located. It is here that he is "introduced" to Toga, the pilot of the Grand Kaiser. Eiji attempts to coax a location for his sister out of Toga by giving a rather lewd description of Aiaka, quote "nice pair of melons, tight butt, what a looker". And he is saying this about his own sister... Toga has no idea what Eiji is talking about, so of course Eiji attempts to beat down Toga and silence him. Eiji flails about as much as he likes but doesn't manage to land a hit.
This is of course when the first Zeravaier attack occurs and, while attempting to escape, Eiji accidentally climbs into the G-attacker, sets it on auto pilot, and is taken directly into the warzone. He is quickly given a crash course in operating his machine and performs admirably considering the sitruation.
Late in the series, Eiji is given command of the Sol Gravion, to pilot it alongside Toga in the normal Gravion.
To put it bluntly, Eiji is usually the lovable idiot. He gets angry a lot, gets assaulted by ther maids regularly, and is constantly at odds with Luna. Eiji does have a strong sense of right and wrong however and his attitude in combat is often against the cold, brutal person that Toga becomes in battle.
As stated, Eiji and Luna are at each other's throats more often than cats fight dogs. One time he even gets pushed through the glass roof directly into Luna's bath. Toga had been showing Eiji around and didn't realize the implications of staring at a naked girl. Luna predictablty flies off the handle at Eiji until Toga intervenes and explians away the problem in his own simple-minded way.
It can be assumed that, at one point or another, that Eiij has a crush on Mizuki Tachibana (not really hard to figure out why) although this is never built upon because they were both too busy trying to figure out what happened to Aiaka to make her dissapear.