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.
With Volumes 7 and 8, there's a Broken Base among the shippers as to whether Sena's storyline is "peaking too early" (in harem series, a sure sign of a Romantic Runner-Up) or not. As of the end of volume 8, the heavy focus on Sena is lampshaded, and volume 9 seems to be leading into a Yozora-heavy storyline.
Foe Yay / Les Yay: There's something more to Yozora and Sena's relationship than their frequent squabbles would indicate. On Yozora's side, she believes nicknames are only for friends, even if it was as insulting as Sena's "Meat". On Sena's side, she confessed to Kodaka that she's secretly happy with her nickname because she has never been given one before. Then comes actual interaction: during the Beach Episode Sena was apparently enjoying Yozora rubbing her back with suntan with her feet, and also commented that Yozora, post-haircut, looks ridiculously handsomeon a male school uniform.
Sena and Yozora are both unpleasant people who don't give much consideration to others, but you can't help but feel really sorry for them every time they accidentally say or do something that highlights that they're just really lonely people who want to make friends but don't know how.
Note that "accidentally" can be something of a stretch of the word. They often know they're being outright cruel, malicious, or vindictive when they say something, they're just unaware that this can come back to bite them. A perfect example is when they first play a galge together... they go out of their way to be mean to one of the girls, and even once it begins to impact the relationship they've developed in the game, they outright refuse to apologize, simply because they hated her that much. And all for no reason other than that she looked "stuck up" to them.
At one point, Yozora gives a rundown of how much fun bullying is. Later, she writes a story where Sena is gang-raped (right in front of Sena).
Sena holds any man who she can manipulate in disdain, and any man she can't manipulate in more disdain, and only seems to like 2D girls.
Yozora's woobie status seems to have kicked into high gear as of the end of Volume 9, as the reasons for her dislike of romance and same-sex friendships are laid bare (her mother's best friend betrayed her by having an affair with her father), and is shown to have a dysfunctional relationship with her sister Hinata as the two took opposite sides in a bitter divorce. Needless to say, Sena's confession and Kodaka hiding out with the Student Council in Volume 8 didn't help, nor did her overhearing Kodaka and Rika's conversation at the end of the volume.
Moment Of Awesome: The OVA contains one of note, courtesy of Sena. As revenge for monsters gang-raping her during Yozora's portion of the relay novel, Sena humiliates Yozora during her part, only to get smacked around and ordered to lick her feet. But rather than comply, Sena steals the flyswatter, hits Yozora, and says, "Like hell I'd ever lick your feet!" before calling her an idiot and running out of the room. It was immensely satisfying to see for those of us who were made uncomfortable by Yozora's part.
Running the Asylum: An incredibly fast example, as the alternate "Plus" manga has already started up, and seems to be written by a Kodaka/Sena shipper. Though it's only begun to be translated, it already seems to be working on turning Yozora into an even bigger bitch than before.
The Die for Our Ship elements are particularly jarring given that Yozora wasn't comfortable with the concept of romance in the first place, and wanted to be just friends anyhow. Thus it was a ship that sunk itself.
Ship-to-Ship Combat: In other parts of the vast reaches of the internet, there's a battle brewing between those who prefer to see Kodaka with Yozora, and others who would rather see him with Sena. The other girls are largely relegated to Demoted to Extra status in this regard. Amusingly, Kodaka himself keeps letting it slip that Rika is really his type (or would be with a better personality, but that goes for all the girls).
If the events of volume 8 go unobstructed, seems like the joke's gonna be on Yozora fans.
Rika and Yukimura's ships seem to be gaining some steam with the second anime season.