This is a "Wild Mass Guess" entry, where we pull out all the sanity stops on theorizing. The regular entry on this topic is elsewhere. Please see this programme note.
Akitsu is/was Sekirei 07.
Akitsu is quite powerful, able to hold her own against Homura (Sekirei 06). She's got elemental powers, which are overwhelmingly clustered towards low-numbered Sekirei so far (03, 06, 08, 09, and 11/12 have them. Possibly 05 as well. The only other ones we know of are 107 and 108). And so far, Sekirei 07 remains conspicuously missing.
As a bonus, since iirc she lacks an Ashikabi, there's a distinct possibility Minato will wing her eventually.
It is impossible for her to be winged, and she does have an Ashikabi(The south, btw), still, she is with him entirely by choice and may pull a Heel-Face Turn.
Akitsu choose to act for the south because he accepted her problems. She also isn't officially in the game, because she has never been winged and Minaka announced that all Sekirei have been winged. And remember, Minato is very special in the terms of the game as an Ashikabi. May be like Seo in that regard as well.
The newest special chapter confirms that Akitsu is Number 07.
Minato and Yukari's father is Minaka and/or Takehito
Pretty straightforward. Minato's mother worked with them (and still works for Minaka), so either (or both) could be the father of her children. iirc, Minato's been implied to resemble both of them (Minaka by Kazehana, Takehito by Miya). The fact that the identity of Minato's father is apparently classified information seems to support this.
Confirmed to be Minaka.
At least some of the information Miya's withholding directly involves Minato somehow
Miya and Matsu clearly know much, much more than the former has currently allowed to latter to reveal, and it seems certain Minato is directly involved with at least some of it. Seo Kaoru, too, knows quite a bit Miya won't let him tell (and the fact that he's got some sort of Anti-Magic just raises more questions)
Possibly, one reason is the students that found the ship were Takami, Minato's mother, and Minaka, his biological father. Another could be that there were more ships with sekirei that landed.
Confirmed to be both. There were eight ships, but the one we know of crashed into the river and wasn't found for a thousand years.
Minaka killed Takehito
Miya's reaction◊ to being asked what she'd have done if Homura had told her about their goal of killing Minaka pretty much says it all. Minaka did something to cause Miya to have that reaction, and the fact that her husband is dead seems to suggest the obvious.
Pretty sure that if Miya believed Minaka to be responsible, she wouldn't need anymore reason than that. However, Minaka is probably idirectly responsible, i.e. he could have taken action to save him, but for whatever reason, chose not to.
Given everything we know about her, if she knew or even had reason to believe Minaka killed Takehito, she would have killed Minaka a long time ago.
It's true that Minaka was not DIRECTLY responsible for Takehito's death, but he was INDIRECTLY responsible because he was doing something with the Jinky that caused them to resonate (which could have gotten a LOT of people killed, as well as all of the infant Sekirei who could not move.) Takehito died because he had to "adjust" the jinky to affect "normal" humans instead, of which he was one, and confine the damage to MBI HQ so he'd be the only one to die.
Higa kidnapped Yukari as some sort of gambit against MBI
Really, he's got to know she's the daughter of Minaka's Dragon, so the only reason he'd be doing something this outright stupid is if he's got something planned. Presumably his attempt to ally with Minato (where he actually asked for Minato's help in resisting MBI, giving some credence to this) was made for the same reasons.
It may also be that he wants Minato on his side, and if he doesn't join willingly, he has a hostage. Shiina is strong, but Minato has some of the strongest Sekirei out there.
Pretty much confirmed, but we may never truly know.
She's the only person we've actually seen Karasuba be nice to. iirc, the best we get with anyone other than Musubi is Karasuba being civil to Miya (who iirc was none too pleased to see her), and that was probably only because Miya is stronger.
All but confirmed in the manga at this point, apparently the only reason Karasuba hasn't gone on an unstoppable rampage to nix all humans due to a bet with Yume over who would win the Plan. Since Musubi is something like the reincarnation of Yume, Karasuba is waiting until they're the last two Sekirei remaining before killing every human being (probably finishing with her Ashikabi, or perhaps using her wish from winning the tournament to do it all in one go)
Minato was experimented on as a child, and just doesn't remember
He caused 6 Sekirei to react with him, 4 of whom are Single numbers, and one of which contains the soul of a single number. His mother is an MBI scientist, and it is hinted his father is too, (Minaka or Takehito, take your pick), he is 19 at the start, meaning that he would have been concieved some time after the ship was found. Conected to a previous WMG, something is being kept from him about himself, possibly this. Also, as Seo has demonstrated, Takehito at least has no problems doing human experimentation.
He is the only one who it has been confirmed to cause a reaction with multiple Sekirei, the others with more than one are shown to take extras by force.
Actually we saw another prior to the 4th match of the third stage. He HAD two, but one of them, "Mimi" was terminated by Higumachi, and both of those Sekirei were indeed winged willingly.
Possibly he is part Sekirei, part human.
Confirmed to be the case with all the modern Ashikabis, and also some humans.
Minato's father is in fact Minaka.
There were 99 unfertilized eggs on the ship. Who are the fathers? Perhaps the reason why Minato is a strong Ashikabi is because Minaka contributed himself to the S-Plan (probably not only him due to the variety of Sekirei). Some Sekirei might be his half-sisters!
Weren't those eggs fertilised?
Yes. Matsu specifically says they were fertilised.
The ship that brought the Sekirei to Earth was a seeding ship
Its purpose was to seed the planet with Sekirei. Most likely it was one of several. Something went wrong on route, causing it to crash land. This may explain the male to female ratio. There is only so much space available, and having more women than men would allow for more births in a shorter amount of time. Because the ship crashed, Miya wasn't woken to begin the process. It wasn't until Minaka boarded that the ship began to awaken.
Confirmed. There were eight ships, but the one we know about crashed into the river and wasn't found for thousands of years.
Minato will be the Big Bad for someone
This may sound strange, but think about it. Minato has it all, he has six Sekireis, all of them are completely loyal (he didn't force any of them). Four of them are single numbers and two of these were in the first disciplinary squad. The other two are Kusano, the last Sekirei, and Musubi, who has the spirit of number 08 inside of her body. He has two of the macguffins, one of which was stolen. He has connections to Yukari, Uzume and Seo, some of the strongest players in the game. Also he is (probably) the son of the Big Bad and The Dragon. He seems much more like a Big Bad then the others.
This theory is likely wishy-washy at best... for the manga. In the anime however, it's a different story, as Minaka seems to be deliberately setting up this sort of situation. One person even refers to Minato as a "boss character."
Gaining more Sekirei increases your brain power
Minato did get all the answers to some questions Matsu gave him without studying. And he has started to lay schemes for battles.
Actually, his younger sister stated that he's quite intelligent, acing his exams through high school. It's just that he's lacking in confidence
Sekirei under go accelerated puberty
This would explain the age discrepancey, and how, in 20 years or less, most Sekirei are adults or young adults, with the only exception being Kuu, the youngest. It would be fine except that 01-05 were all awake and adult when 06 and onwards was still asleep and presumably a chibi.
Whether this growth is artificial or natural is debatable.
Minato is a "tuned" human — Sekirei's version of a Coordinator
Containing the DNA of both Takehito and Minaka. When she found out that she was impregnated by Minaka, Minato's mom altered the embryo using Takehito's DNA. This would explain why Minato is said to resemble both Minaka and Takehito— his paternal genetics are half-Minaka and half-Takehito (or whatever is semantically accurate).
Takami is not Minato's biological mother.
Despite the fact that Takami and Minato do share a familial resemblance, it also occurred to me at how certain blatantly unrelated characters in this series do as well. Then I started thinking of how oddly Miya acts around Minato, especially the lengths she goes to in order to keeps secrets from Minato. I've also noted how many times those secrets seem to relate to Miya specifically. So the random thought came into my mind: What if Miya is Minato's TRUE mother? Minato is sort of barely young enough to be Miya's kid, depending on how soon she woke up and how fast she formed a bond with Takehito, the timeline is tight but possible. The reason for keeping it a secret is that Minato is a happy, well-adjusted individual when all is said and done, and the Sekirei plan does take its toll on him. So Miya not only would not want to put further undue stress on him, but doesn't want him lashing out at Takami either. It would also explain, if my memory serves correctly, why anybody would be performing tests on Minato specifically (at such a young age anyway). It could also give Miya additional reason for wanting to kill Minaka, as it is likely Minaka would ultimately, if indirectly, be responsible for both the death of her husband, and the theft of her son.
Miya is cock-blocking Minato less for comedic value and more for prevention of heart break.
Okay, we have all but confirmed that Sekirei and Humans are genetically compatible enough to reproduce with one another, since modern Ashikabi are descended from the original Sekirei and Ashikabi pairs. Knowing this, and knowing Miya's personal loss of HER Ashikabi/Husband, it's safe to assume that she knows deep in the bone how real a chance Minato's harem has of all getting killed off. She's constantly cock-blocking Minato and his girls not because it's a hilarious gag and it neatly avoids the censors coming down on the series like a ton of bricks, but because she probably knows how much MORE devastated Minato would be to find he's got a kid on the way, who suddenly is dead as well due either to the sheer amount of punishment Sekirei's undergo during battle (regardless of victory), or because the potential mother is dead after losing a match. Technically this should go in Fridge I guess.
Also, several of the women have shown very serious Clingy Jealous Girl traits, and Matsu even went Yandere briefly on Musubi with a Kill Sat (but now she mostly just laughs at the other girls fighting over him). Its likely that if Minato did sleep with one of them, the others would get insanely jealous and probably destroy Maison Izumo in the process (especially Tsukiumi). Which would really piss off Miya, especially if it happened while she wasn't home at the time. If he manages to find a new place to live, on the other hand...
Sekirei with close numbers tend to be from the same genetic pool. In other words, they're genetically siblings.
This is the reason you find that sequential Numbers tend to share not only similar features, but closely-related abilities. Karasuba (#04), Mutsu (#05), Homura (#06), and Akitsu (#07) have very similar features and coloration. Hikari (#11) and Hibiki (#12) are generally referred to as being twins, and share the same powers. Mitsuha (#38) and Mitsuki (#39) also appear to be twins, with only a slight variation on their weapon of choice (whip vs. wires).
This isn't so much Wild Mass Guessing as it is a statement of fact.
The last few Numbers are so powerful because they were being tuned during the Jinki Activation incident
It is noticeable that the Sekirei #104 - #108 are all extremely powerful, on or close to equal footing with the Single Numbers. In chapter 139, we are told that several Sekirei were being tuned at the time of the incident. One of the chambers visible is labeled #105 (Benitsubasa), with at least three or four other chambers present in the room. Haihane, Benitsubasa, Shiina, and Kusano have all been shown to be extremely powerful, able to keep pace with even the Single Numbers. The Jinki are able to influence the core of a Sekirei, and while they were being adjusted, one of the Jinki was accidentally activated. This could mean that they were somehow effected by it, giving them greater potential power than they otherwise would have possessed.
Longterm exposure to the Jinki and other Sekirei-related energies causes a human's hair to turn white.
This is why Minaka, Takami, and Takehito all have white hair. In the manga, Natsuo is also shown to have white hair.....meaning that every single prominent employee of MBI has the same unusual hair color. Two of them, Minaka and Takami, are already established to have originally had black hair before being Locked into Strangeness. Since the other two named employees both have white hair, it's possible that something about the energy given off by the Over Technology causes this over time.
At least, the majority of characters are. Natsuo is clearly Straight Gay, but almost every other character has at least one moment of Ho Yay, Les Yay, or Stupid Sexy Flanders. Kazehana at one point states that worrying about gender when it comes to love is an "old-fashioned" attitude.
Sekirei and Heaven's Lost Property are in the same universe.
It would turn out that the Angleiods were created by Minaka and/or his descendants and his inspiration came from the sekirei. Also Akitsu waould be the first Angloid prototype, what with the "yes master" and such.