can someone spoil me a bit on how dark this manga is?

for example there is "traumatic past" who experienced and what happened (just use spoiler tags)

also Deaths, so are there like main character deaths?

It's borderline seinen/shounen. "Darkness" here is comparable to manga like Claymore or Arago. Characters does die, and doesn't resurrect, people are greedy and does dirty things to other people, crime is a main part of the story - main cast are members of private police force. On the other side, it's a shounen manga - about first love, school problems, power of friendship and determination etc.

One of main characters die in third or fourth volume, another has mental problems. Minor characters die almost every volume, there is attempted rape, torture, drugs etc. but nothing graphic.

Biggest weakness of this manga is main male character suffering from light version of "Shinji Ikaru syndrome". He starts as a typical shounen manga weakling, is used as "damsel of distress" in first half of the manga. Although he grows considerably stronger later, many readers still consider him as nuisance.

It's borderline seinen/shounen.

It draws from typical shounen and seinen cliches.

There's no such thing as a borderline demographic, comrade. Only stories that attempt to transcend the intended audience. Like Animaniacs.

"I found Prince!"
"No, no, no... FINGER prints"
".........I don't think so."

Good night everybody? Yes indeed.

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blacker than the biggest blackest boner alive.

Dark, dude.