Oh, come on... Aika is what it is! It's supposed to be fan service and it accomplishes the goal quite well.
I'd give it a C grade rather than the D- you gave it. The story, albeit highly improbable and sometimes downright cliche, was written to support the time necessary to fill the episodes with as much panty shot martial arts as could be written into each episode. The story was a vehicle for fan service, rather than the otherwise trite way some directors cut their other anime. And it's refreshing to see it.
To be honest, Aika didn't have lot of grab on me. The supernatural being she becomes was just a way to show a dumbed down tentacle sex scene without the penetration or blatant use of other borrowed anime plots. The villian girl was cool and all, but I never got a sense of the importance of the villiany anyway. It was like looking at two small college football teams play and saying "ok, who are these two teams?"
Fan service is where it's at. It may sound juvenile to say, but who wouldn't show some sexually charged fanboys the anime version of an up-close panty shot 200+ times in a story? Sex sells, and what also sells is the partial nudity shown in Agent Aika.
I say, "lighten up" and get your kicks out of an anime that cares enough about you to show you the lovely white innocense covering the target of a male's biological intrigue.