I'll have to go for overview here, because I read all of the volumes in one go again, so the details become a bit mushy.
"The heroine, our middle-school baseball obsessed shounen, Haruna has always loved romance shoujo manga apart from baseball. She put her all into her sport in middle school and now that she's in high school she has vowed to go after a boyfriend just as much as she ever did sports. And because she knows how to go after her goals in sport she uses the EXACT SAME WAY of researching and doing her utmost best and being totally focussed (and therefore intimidating) in trying to reach her goal - and scares the boys off.
By chance she overhears some boys saying that the girl-avoiding (he actually has a believable reason for that) Yoh has a lot of savy about which girl is popular and why. He also has great style himself. When the two bump into each other the second time after she has realised that her feel for the "game" of "getting a boyfriend" just isn't up to par and she badly needs advice, she forthrightly asks him to be her teacher in becoming attractive to boys!
Yoh not being an idiot, says no. He does take note of her determination, though. Yoh's sister Asami thinks the whole situation too funny and promises to help Haruna, but since she's fairly self-centered and only does it to amuse herself, Yoh realizes that the grateful Haruna is unlikely to succeed (and he feels a bit guilty, because he actually has a conscience, unlike his flighty beauty of a younger sister). On a video that Asami is watching, Yoh sees Haruna's last, lost baseball game - how she did her best, how she cried her heart out when it wasn't enough - and that makes him change his mind. She's not like the girls he's had the bad experiences with before, and he wants her to succeed.
But one thing is made absolutely clear: She had better not fall in love with HIM!"
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In this second volume (with a very untypical Yoh backcover and a very typical busy front cover - typical for this series, Sensei! looks a lot different. One thing the incredible emphasis on cool "current" outfits for girls and boys in this series means is that the chapter intros look incredibly dated or weird in some way, heh) Haruna isn't quite sure yet if what she feels for Fumi is love (after all, she's never been in love before) but Yoh encourages her exploration of this (he knows that Fumi is a kind and forthright guy and can be trusted with Haruna, and he hasn't had a girlfriend yet either).
There's a lovely bit of Yoh coaching Haruna on their first date (Fumi isn't aware that it is a first date, as Haruna hasn't confessed yet but he cheerfully has fun with her at an amusement park, because she's fun and he likes her as such and Yoh asked him to go) via text messages (yup, it's a series after the 2000s for sure, actually uses mobiles quite a bit).
Both Fumi and Haruna are honest, cheeful, direct and emotional - and they have the time of their lives, but Asami who is starting to resent the time that Yoh spends on coaching Haruna (a) she's selfish, b) she has a brother complex which will rear its ugly head whenever Kawahara-sensei deems it good to remind us, c) she's envious of Haruna's direct and kind nature, I think) - even though she was the one who started it (because it would be "amusing") - doesn't like how well everything is developing and that Yoh tells her she can't take part in the scheme because she would screw it up (Mr. Diplomatic Yoh isn't, especially not when dealing with Asami).
I REALLY like this part, although I cry my eyes out when I read it: the clear indication that Fumi and Haruna would be totally great for each other, even if their relationship would develop really slowly because they are both not about the subtleties at all. You can almost taste that Fumi would be surprised and happy to be confessed to by Haruna and would swear to do his best by her, and she would swear it back and they would do their best by each other never feeling any pressure from outside, because both would never conceive of either of them being tempted by anyone else cause that is not how they work....
So the first date is a success with Fumi and Haruna making their small step of exchanging phone numbers to start building their own connections outside of Yoh & friends. But when Fumi drops by Yoh's that day to hang out and he isn't there (he's still doing a play-by-play review with Haruna of how it worked and Haruna is sure now that she's in love with Fumi), well Asami is there...
And uses her very well-honed powers to seduce an astonhised Fumi into becoming her boyfriend, so that when Haruna and Yoh show up (Haruna wanting to share her happiness with her "encouraging new friend Asa") they find Asami on Fumi's lap. And Haruna's world basically crumbles. At that age, after such a high and with her nature, totally understandable.
Haruna cries a lot in this series, but she never comes across as a crybaby to me, because she's so intensely emotional about the things and people that interest her (also, I cry for relief from emotional tension and in sympathy easily, too ^^).
And you know, because Fumi is so oblivious and didn't realise he was Haruna's object of interest, he's just totally happy that Asami confessed to him, which makes the situation doubly horrible for Haruna. But while she is very direct, Haruna is not stupid - she knows that she told Asami that she liked Fumi, she knows that Asami said she had no interest in him, so why this?
I think the betrayal from Asami's side is far more cutting and effective than losing the small blossom of possible love interest. I do resent that Kawahara-sensei keeps using (of the two important recurring female characters in this series) Asami as the selfish, beautiful, spoiled b*tch. Not every pretty girl is that.
And Mami? Haruna's old friend whom she looks up to? Only ever shows up to give her tips when her love life goes wrong. That's another thing that KnT gets right - female friendship is not ALL about boys, there are other interests in a girls' life.
Yoh who totally sees through both Asami and Fumi now can't help but want to make Haruna feel better - he calls them both out, by the way - one is clueless and Asami doesn't care. And when he does go to the trouble to comfort Haruna in turmoil the seeds for her feelings for Yoh to go from coach to wished-for love interest are laid. And, personally, I've also managed to move forward from a crushing situation by having an extended good cry ^^.
There's an interesting coda to the Fumi/Asami situation that while Asa is all that I said above, she also has a conscience. She just usually manages to ignore it handily - so she maneuvers Fumi into waiting for her outside in the snow for hours and then contacts Haruna (who has no idea why) - most probably in her oblique way to give Haruna the opportunity to save a frozen Fumi from his fate - totally ignoring the fact that Fumi (like Haruna) is as faithful as a dog and would not - now that he has decided to be her boyfriend - develop any feelings for Haruna (he might appreciate being saved from the cold, though ^^).
Or maybe she hopes (more likely) that Haruna will explicitly give her blessing to Asa/Fumi, so she won't have to feel guilty anymore. Haruna has no idea why she should think that Asa is a bad person (she's just not built that way) but she CAN point out to Asa that Fumi isn't Yoh, so you can't treat him with subtlety so Asa better had go to Fumi now. Whatever it is, Fumi sees the two girls in the restaurant and comes in from the cold himself.
Neither he nor Haruna blame Asa (*grrr* but consistent with their personalities) and so she can now be happy with Fumi.
The rest of the volume is concerned with Haruna realising she has fallen for Yoh and dealing with the fact that a) she wasn't supposed to and b) she is nowhere the type of girl he used to fall for. Meanwhile Yoh gets to see a sporty Haruna in her element and both learn to appreciate each other more.