Well I feel sad for Atsuko and Yuko, but I don't think Kai and Haruna is at fault too.
I mean the setting of the stories is that they are very popular people that have lots of fans.
Fans can be good and bad, right?
Some more they are on a relationship before Atsuko and Yuko meet Kai and Haruna... so for Atsuko and Yuko to fall in love with them, weren't they fault
at all.
Also their friends keep teasing them (AtsukoxKai and HarunaxYuu), they are also at fault to keep encourage Atsuko and Yuko with their feeling for Kai and Haruna respectively.
I mean Kuu and Mariko knew about them, and they are the ones that teasing them more... I can say the faults lie with them some of it.
For Kai and Haruna, telling people their love lives or not.... I think I can respect their choice... I mean it's their loves lives...who are they so say anything. It's their own business. Some more, they might need more time with their relationship... I mean they are new into the relationship... so it might not be stable yet... they might need more time to be more comfortable with their new status.
As I said before, they are very popular... so before others giving them hard time whether in or out of school.
Furthermore, in the story... it seem like both Atsuko and Yuko... the first time they met Kai and Haruna respectively... they are already falling in love with them so whether they know the relationship before hand does it matter? They are going to be broken heart anyway.
Okay may be knowing the relationship before seeing them making love to each other might soften then blow but are they at fault... may be a bit.
I can't really say Kai and Haruna 100% at fault... everyone contributes in this tragedy.
In the other hand, Sae is very much at fault with what he did to YukixMayu relationship. He tried to blackmail Mayu... Mayu please quickly think... To solve your problem is very easy. 1st, tell Yuki what Sae said (about the black mail), 2nd, quit and find another job... there is always some where you can get a job. Doctors... there are always places that will need doctors. Or 3rd, Tell Yuki to ask for a break until Yuki graduate from the Uni... then continue the relationship.
But these are all my personal opinions, I don't mean to offense anyone with my comments.
I hope u guys don't mind them.
It's very good fic. Thank you for the update.
Can't wait to see the next one.