This is much later than other comments, but I feel I should make it anyways to prevent misunderstandings for those reading spoilers from other reviews. Most importantly,
For clarification, Sakashita Kanori does NOT end up with Ena. She meets someone that has a similar hairstyle with Ena (but completely different personality). But yes, she does not end up with Kouta. As BimboSilly says, the two original pairings don't work out and the girls find other guys to be with.
So, at some point in v12, Kanori is dealing with family issues (the same old divorce shit, mom wants her to move with her elsewhere) and bumps into Ena visiting Sakagami Kazumi, who is finally starting to realize how messed up her boyfriend is and in v11 broke up with him. Worried due to the abusive nature of the relationship (especially since in v11 he forced himself on Kazumi when she wanted to break up), she also follows to Kazumi's home. When Kanori is resting in the living room and Ena is trying to force himself onto Kazumi (who is now not so lovey-dovey with him anymore and thus is unwilling), Kanori notices this and barges in with scissors. At some point, Kazumi finally stands up for herself and asks Ena to leave, officially ending the relationship completely.
Kanori and Kouta's relationship is going fine, including some emotional support for Kanori's family matters. Then there is this girl that seems to barge into Kouta's room, acting like a stalker, using the key she stole from Kanori in v11. I'm not sure what specifically her relationship is, but apparently she was a middle school classmate and likes Kouta. After Kanori and Kouta's New Years Visit (I guess), he comes home to find her there. She tries to force herself on him, he pushes her away, Kanori visits, Kouta tries to hurry her away outside but fails, and finally out of revenge the girl tells something to Kanori that makes her break up with Kouta. Not sure what it was but it was probably a terrible enough secret (probably from what Kouta was the same as Ena before) that makes Kanori's trust in Kouta fail miserably.
v13, Kanori starts avoiding Kouta. Kazumi sees that Kanori is depressed. Ena somehow starts to miss Kazumi (and well, fuck him, because all he does is use his groin to think). After she meets him again, she uses this opportunity to try to get Kanori and Kouta together again by having all 4 of them visit an amusement park to go down memory lane (no luck). Later, Kanori moves away.
After than, about 4 months later where Kanori transfered to, a boy named Okuno (if I'm reading the hiragana おくの correctly) seems to be friendly with her, and it seems he actually likes her. He invites her to the fireworks, which she agrees to go (and does in her yukata). While watching fireworks, she cries remembering the past. The final chapter (v13c56) is her visiting her hometown and meeting up with her friends (including Kazumi) and happily playing around. Kazumi has Kouta go to a place where he would bump into Kanori, and they see each other but Kanori still can't accept him. She returns home where Okuno is waiting for her and he offers to walk her home when she is depressed. On the side note, Kazumi seems to have started to get closer to a workmate of hers.
But anyhow, I don't like any of the characters except Kanori, who I feel doesn't deserve all the crap pulled on her all the time. And though her personality is a complete doormat, I just plain feel sorry for Kazumi, who is constantly emotionally cornered by her boyfriend into this messed up relationship due to her devotion. Really, I'm happy that both of them discontinued their messed up relationship with the two male leads, even if Kouta wasn't all that bad for the duration of the story (though I can't speak for his past). Ena is a complete jackass that I wished he got some punishment, while ironically it was Kouta whose past came back to haunt him.
Anyhow, a screwed up story about terrible relationships. A somewhat cynical view of real-life relationships when things go all wrong, I guess. A bit too long for everything to play out the trainwreck, and its quite a depressing trainwreck.
EDIT: Going back a bit through the RAWS, if I'm understanding correctly:
Kouta sleeps with that stalker girl (end of v7c27, hinted at in v8), hence cheated on Kanori and hence her inability to trust him anymore due to him being the same as Ena in her eyes. So justice served because he doesn't have a happy ending, although Ena is far worse so it's somewhat unfair.
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