These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
#84 Yashima and Junichi, the abused Sekirei. Some theorize she may be Too Kinky to Torture, and was legitimately winged by her Ashikabi.
Mikogami is Obliviously Evil. He approaches the Sekirei Plan as a very exciting game, and concerns himself with things like following the rules and the best ways to keep things interesting, even if doing so puts him at a disadvantage. None of his actions violate the rules, and in fact his aggressive pursuit of new Sekirei for his collection is not only allowed, but actively endorsed by Minaka when he runs the "Green Girl" and "Final Feather" events. Furthermore, his interactions with his Sekirei and anger at the mistreatment of the East's Sekirei suggest he's a Benevolent Boss.
Awesome Music: "Change the World", the insert song that plays during Musubi's battle with Karasuba in the 2nd season finale.
Base Breaker: Kaho, the #87 Sekirei introduced late in the manga, has become this recently due to her Spotlight-Stealing Squad tendencies at the start of the 4th stage of the Sekirei Plan when she encounters Karasuba. Almost everyone believes that she is nothing but fodder to hype Karasuba's power to make her eventual fight against Musubi more meaningful, yet the manga still ends up spending a great deal of time developing Kaho's character at this point. While some like that Kaho is being developed, others wish for the manga to simply get her character out of the way already to move on to the more important plot-points of the story. Which does eventually happen as the way the fans guessed it would.
Broken Base: Besides the above-mentioned Kaho, the Character Development of Mikogami is a source of controversy among fans. The revelation that he cares for his Sekirei, and his numerous Pet the Dog moments with Akitsu either make him a complex character or strike fans as Jumping the Shark by suddenly making him a sympathetic Anti-Villain. Akitsu's winging was equally poorly received by fans that had hoped to see Akitsu join Minato's harem.
Crazy Awesome: Minaka always had shades of this; his reason for creating and enforcing the Sekirei Plan is one part Science, one part Godhood and two parts for shits and giggles, but when he blows up his own building while still inside of it to make the game he was playing with Minato more interesting in the anime, he really does show his Crazy Awesome side.
Mikogami, since he's an obvious stumbling block for Akitsu joining Minato's harem. Discussions of his impending death, and Akitsu running off to join Minato's harem, occur literally almost every single chapter.
Evil Is Sexy: Yukari is literally dumbstruck upon first meeting Higa, who does a sexy casual pose on the hood of his Cool Car.
Fanon: A popular theory is that Akitsu will be winged by Minato, and join his harem. Often coupled with a hefty dose of Die for Our Ship. Ends up being Jossed, as she's eventually winged by Mikogami prior to overloading.
Fan-Preferred Couple: Minato x Akitsu is popular, with fans hoping that she would end up joining Minato's harem.
Fridge Brilliance: Mikogami's development was a source of Broken Base, but later chapters reveal the bond with a Sekirei can naturally change an Ashikabi for the better over time. This is driven home when Natsuo realizes that Minaka intentionally gave him Benitsubasa and Haihane in the hopes their bond would make him realize You Are Not Alone. It becomes even greater brilliance when one recalls the Japanese legend of the Wagtail that Yume repeatedly references — they are the birds that supposedly taught the Gods about love.
Game Breaker: Karasuba states that Miya and Yume are both this.
Minato. 6 Sekirei, 5 of which are among the first ten - and thus stronger than the other Sekirei as a result of not having any inhibitors (namely numbers 2, 3, 6, 8/88 (Yume and Musubi share a body), 9, and 108.). Number 1, the strongest Sekirei, is his landlady and ally, and she's actually not part of the Sekirei Plan. In short, he not only has a large number of Sekirei, but more than half of the strongest ones are on his side.
Growing the Beard: The series started out primarily as fanservice and comedy, but has developed into a more serious, character-driven story over time. It still delivers on the fanservice, but has a stronger focus on plot and characterization. This began with the Bridge Escape story arc, and has continued since then.
Karasuba, who aggressively pins Musubi to a wall and commands her to get stronger, with the handle of her sword poking Musubi in a very suggestive fashion. It's also implied that Yume is The Lost Lenore, in a very disturbing kind of way.
Ron the Death Eater: Musubi seems to be the least liked of all of Minato's Sekirei by fanfic writers, who often give Minato literally any other Sekirei but Musubi in their new harems for him, with some stories actually having her murdered to justify her not showing up in said fanfic. The main reasons for this stem from how "obvious" it seems she'll be the "winner" of any battle for his heart, and since that presents a major stumbling block to shippers, she gets written out or is often NOT the first one to meet him so fic writers can keep Minato's options open. As for her as a character, she does tend to get seen by some a Satellite Love Interest compared to the other Sekirei (in their defense, she can be rather ditzy), so that's another reason.
Shipping Goggles: Not as many fans as Akitsu shippers, but Uzume does have her fans who want her with Minato.
Minato's little sister, Yukari, and her #107 Sekirei, Shiina, get this reaction when it became clear to the readers that the writer had purposefully put the two on a raft to escape the city so that they wouldn't have to be dealt with in the next stage of the Sekirei plan. Doesn't help that at this point in the competition, Shiina was established to be quite an overpowered Sekirei.
Akitsu gets this reaction from many due to not joining Minato's harem, but mostly because she gets killed off shortly after she finally gets winged.
Too Cool to Live: Uzume, who inspires Minato to step up to the plate in the hope that he can restore her after winning.
Karasuba is female too. The only reason its hard to tell at times is because unlike the other women she usually doesn't go out of her way to show her femininity off in public and has a fairly gender neutral voice.
Villain Decay: Benitsubasa and Haihane suffer what is possibly the fastest villain decay in anime history, portrayed as terrifyingly powerful antagonists at the end of season one, but reduced to incompetent jokes by the start of season two.
Woobie Species: The Sekirei, as a result of being forced to participate in a "game" that forces them to find their One True Love.....and then fight to the death with the rest of their kind in the distant hope that they might be the Final One, and earn the right to remain with their Ashikabi. It becomes even more cruel, since the odds are stacked against the vast majority thanks to the Single Numbers being more powerful than normal Sekirei. This drives several pairs to attempt to escape the game, only to be hunted down and slaughtered by the Discipline Squad.