"Everyone" has a past; Reflecting on Kai, parallels between him and Misaki
Spoilers ahead.
“She should quit. Anybody who wants to quit should quit.”
“Using that as a perpetual reason for your own weakness is childish, pure and simple."
At face value, this is probably the cruelist thing Kai has said in the series. And naturally, a lot of people were upset with him for wanting Misaki to quit; looking back now though, this line comes off as indignant--and maybe just a touch jealous.
Four years ago, Kai also lost his parents in a sudden and inexplicable accident, and unlike Misaki he had no close support base to console him. While she had Shin, it's doubtful that he was anything but emotionally distant from his uncle's family. Kai had to deal with the lingering feelings following their deaths alone and without any of the economic guarantees that Misaki's inheritance1 of her parents' shop brought her. His future was completely uncertain.
In fact, the massive disparity in time between Kai moving in with his uncle and meeting Ren(four years versus just under two years ago) suggests that he had gone on like that for a while. What's more, once Kai started making friends again and opening up to people, those friends were taken away from him in a way that Misaki's never were. Kai got over one tragedy, only to have yet another sudden and inexplicable accident kick him while he's down. Ren literally walked away fine one day, and started hurling Tetsu across rooms the next. The opaque nature of PSY Qualia, and Kai's own sense of guilt about something which he had no way of fighting back against, left him powerless.
"Everyone has a past" is coming from someone who knows that the Foo Fighters are out there, and have already won the national championship once before. Yes, Kai is being a hypocrite by criticizing Misaki for being weak when he himself abandoned Ren, but he also endured far worse than her before he gave up. I'm not saying this in favor of excusing his own poor treatment of his teammates, but "she should quit" is pretty sound advice when you know ahead of time that things are only going to get worse once the national competition rolls around.
Much worse.
1 Various translations call Shin either her uncle or cousin. Both of these pin Misaki as the target of her parents' inheritance, once she comes of age.
