This is the kind of manga, that story anchor a LOT on past there's a manga like that but good author usually realize that it wont make the story good so they will abandon that secret past and move on, but some(a lot) are not good enough to do or even realize that fault, Code breaker is in that category.
First of all, the whole secret past and amnesia is stupid not only its strange how miraculous it is that both of MC are not remembering anything, the other character who know also trying to hide it without any good reason, seriously this author is desperate to hide thing just so the series can stay longer. This make the story extremely boring and when they reveal it, it doesn't make thing better because you know what it just didn't make sense for any of them to hide it, and the MC also doesn't seems want to try to pursue it.
Next was the character it was all good in the beginning the guy was cool, and the girl was innocent so its normal when you don't want someone killed, but later on it get annoying the girl literally forcing her ideal to other while she never once try to understand others, I meant her friend almost get killed by bad guy and yet she say she doesn't want the bad guy killed and just let them go, wow if I have friend like that I'm gonna leave them in heartbeat she doesn't care about friends near them only care about the bad guy. Other worse thing is enemy/ally can switch so easily and get away with it, and other people just have to put up with it, it was so annoying.
Later the character progress, the girl pretty much stay the same, the guy became mushy, buhuu I can't be with her because I'm evil, oh god get over it okay.
For other aspect everything was average just like typical shounen other people who died are just a fooder, if the character has been in 10 or so chapter then you can be sure it will never died.