I want to start off by saying I love stories featuring characters with any sort of mental illnesses because it adds a great depth of conflict. Now obviously the "split-personality" is not a very new concept, with its very first debut in "The Strange Case of Dr. Jekyll and Mr. Hyde". There's been a lot of representations of Dissociative Identity Disorder, which I'm not really that critical about because I can suspend my disbelief for enjoyment. I've seen movies and shows which dont potray the illness accurately but I still enjoy it because the conflicts the characters go through are still relateable and have depth towards them.

WARNING MAJOR SPOILERS AHEAD:

So when I looked around for manga that had this as a part of a conflict, I was naturally intrigued. During the first arc where the MC, Eiji Urashima (the cheery one) was discovering the existence of his alter "B1", was great and also pretty typical. We also learn about how Eiji's biological Father is a serial killer named LL who brutally tortured and killed young girls. Already adding a bunch of conflict which I personally liked. The first arc was basically, the whole mystery around why B1 emerged to interact with gangs, and it was really fun to see the dynamic. Most of the characters were pretty say generic as there wasn't much revealed about them YET.
During the aftermath of the arc, it was revealed that Kyouka, Eiji's girlfriend only got along with him because she knew he was LL's son and admired LL since she murdered her big sister, who "supposedly" caused her to be abused. She's revealed to be the one that killed B1's "mole/rat", showing her twisted nature. This was when I started seeing the flaws within the manga. I think it would've been better if they even subtly gave some foreshadowing as to her true nature instead of keeping it completely hidden. It would've made the reveal much more well fleshed out and it just felt like the author just came up with it on the whim.

Then we're stuck with B1 as our protagonist for the remainder of the series. A douche who is very unlikeable and the complete opposite of the initial half's MC. The manga would've been a lot more unique or dynamic if they had the two alters interact with each other in some way, because we never actually see them converse together (not even through notes), and it just completely wastes a potentially incredible character plot with it. Anyway near the end of the manga, it is revealed that Eiji's adoptive father is the true LL who framed Eiji's dad and only did the murders because he was a psychopath. No hints, no foreshadowing, nothing building upto this whatsoever. It's just dropped like that. I had my suspicions on the guy ever since it was stated that he was the parole officer for the "accused LL". But goddamn it really shows that the author never really planned any of this from the start. Plot twists are made just for the sake of it, with no proper preparations or care towards it.
In the end, it is shown that Eiji Urashimi who was thought to be "killed" was probably inside him somewhere still alive. And then there's the whole B1's relationship towards Eiji as stated before has no real emotional impact or make any logical development on the character because their interactions are subpar. If they compensated this by at least putting notes/videos to interact with each other, or even tried to make it so that the other alter were aware of what was happening while one was in control and actively was able to communicate would've made Eiji's "death" a lot more impactful to not just the audience but to B1 as well. Forcing actual character growth and not some "If you kill LL you become just like him" (which is a pretty bullshit ideology at this point lets be clear)