The main character (Sosuke) is a socially inept, black-haired school-age guy. There's romantic tension between him and the two main female characters who are both his age: one of whom (Tessa) has a quiet, soft-spoken personality but shows steely resolve at times, the other of whom (Kaname) is loud, fiery, overreacts and always calls him a baka or hentai, but everyone suspects she has feelings for him.
The main character, as well as being a schoolage boy, is also an operative for a secretive military organisation, and the best person in the world at piloting a unique mech whose power is not fully understood (much to the envy of one of his schoolfriends, a military otaku). The quiet steely girl is also at this military organisation - in fact she's been there for longer than him. They seem to hire schoolkids, for reasons not fully explained.
The third main female character is Melissa, a woman in her twenties, also working for this organisation, who dresses skimpily and drinks beer to excess. But she has a good heart and is committed to serving militarily as best as she can (as is her associate in the organisation, Kurz, a male playboy type about her age, with long hair). This woman and the fiery-tempered girl are the main focus of the series' light fanservice, although the quiet girl is also shown near-naked quite early on...
Of the two school-age girls, the blue-haired one so which one's that? is introduced in episode 1 as a main character, the other only around episode 8 (of 26).
Alex is trying to make the description fit both NeonGenesisEvangelion and FullMetalPanic. The problem is, the girls don't quite match - the blue haired one in NGE is the quiet one (Rei) and in FMP, the blue-haired one is the loud one (Kaname). Thus he uses unclear wording. It's quite clever, but if you haven't seen both series, it is confusing - SunKitten
Contrast NeonGenesisEvangelion. Originally a novel (there are now several), then a manga serialised (still going as of 2002-03) in Kadokawa's Comic Dragon magazine: