I notice that a major criticism a lot of people have of this manga is the amount of character deaths. And while I do get it -- I fucking hate that Bomb and Sharkboy are just gone now -- it works a whole lot better for the actual MCs left.
Everyone that dies usually have death flags waving about anyway. The more time we have committed to learning a character's past, the least likely they are to die. If they do die, then it's meant to fuck with you if you became attached to them.
As for the ones who don't have time devoted to showing their character, fuck 'em. No, really, I think that's actually what you're meant to do as a reader of this story. That sucks, but rampant chaos and uncontrollable death seems to be a running theme of the mangaka, what with Fire Punch and all.