What makes the dead boy detectives group seem so close knit and real is that they all have subtle links to each other.
Charles and Edwin is the most obvious, they're both ghosts, both died young, were killed by bullies and have been friends for years. They're the only people who can truly relate to each other regarding those things.
Charles and Crystal are linked (at least this season) through the disconnect between them and their parents, most shown in the scene where Charles shows Crystal his parents through the mirror and she opens up to him about missing her own as she can't remember them. They also seem to deal with hardship the same way, pushing it down, searching for a distraction or getting defensive.
Charles and Niko are linked through being the happy, positive friend yet actually having a lot of grief and pain under the surface. For Charles that's to do with his abusive father and his death, and Niko it's losing her father and the loneliness that caused her.
Edwin and Niko are linked as they feel like two halves of a whole in terms of the way they think, Niko is a lot more to do with emotions and Edwin is more logical. They complete each other in that way. They also both have a link through needing courage, Niko looks for dandelions for it and then gives Edwin the red sea glass for courage.
Edwin and Crystal are pointed out by Charles to be very similar characters, they're both headstrong, stubborn, plain mean some of the time. It makes them clash but as the show progresses they do show more understanding in each other. They read to me similar to a sibling relationship, they argue just as much as they care.
Crystal and Niko (like Crystal and Charles) bond over missing their parents, and as surface level as it sounds, them both being human balances the group out, the same way Charles and Edwin both being ghosts is part of their link.
It's like they were all made for each other! and being in a show they literally were, but it never feels forced or artificial or even all that obvious why they all click so well as a group. but they do!!
