The first is Agent Aika. This one has 7 episodes each being about 25-29 minutes long. This series came out in 1997-1999. And it has an english dub.
The second is AIKa R-16: Virgin Mission, which has 3 episodes each also being 25 minutes long. This series has the same Aika of the first one, but one shows her when she was 16 years old. This came out in 2007. This one also has an english dub.
The third is AIKa: Zero. This last one also has only 3 episodes and again they are each about 25 minutes long. This final part came out in 2009 and shows the Aika when she was 19 years old. This is the only one of the three without an english dub.
We shall start with Agent Aika. This is the story of Aika as an adult who salvages objects from the ocean floor for those who will pay for them. As all of Tokyo and more of the world is now deep in the oceans this job has gained quite a bit of competition. She gets hired for a job and ends in a whole conspiracy of world destruction, followed by rebirth and repopulation with a ship full of beautiful women. Aika and her friends must fight them to stop their nefarious plans. But wait, Aika also has a that super piece of clothing that is really a metaloid being that changes the very chemistry of her body. It strips her down then barely wraps itself around her private parts. It then gives her strength and agility beyond the norm. She uses this to aid her and even save her life at times. But wait there is still more. The Harem organization continues fighting Aika, even after their original goal was thwarted.
This high school aged Aika wants adventure, and loves submarines. She gets her scavenging licence and then makes some new friends (rich friends that is) who also want adventure. They set off on a quest to find out the meaning of a secret code butterfly tattoo that leads them to fighting mind-controlled classmates and clones. And their teacher turns out to be a bikini-clad bad guy behind a lot of this whole thing. Unlike the first series the men don's seem to be as affected in this one by the sexuality all around them. But this may be due to the fact that because most of the girls are 16 or so, it would be too perverted to do so.
Finally we see a 19 year old Aika. This one starts with a school conspiracy and weird glowing tentacles that seem to take possession of beautiful young girls. This one also has quite a few lesbian jokes and insinuations. Aliens and a whole bunch of penetration jokes. This one tells the tale of how Aika fights to discover the truth of a high school conspiracy and save her friends.
Honestly these were rather boring series. I only finished them because of how short they were. The only one with a real semblance of a somewhat good story arc was the first, but the dialog was terrible. The last one had the best video quality and dialog, but has a rather stupid storyline. The middle one Was kinda just between the two in all regards. It wasn't the worst at anything, but it wasn't the best at anything either.
So here are the scores. Agent Aika receives a 5.5 out of 11. AIKa R-16: Virgin Mission gets a 6.5 out of 11. And AIKa: Zero gets a 5.5 out of 11.