Politically, socially and philosophically (if you want to call it that way), BEM fails to create the serious plot that it wanted.

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WARNING: SPOILERS

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Remember, children. Showing vague social stereotypes isn't good if you're trying to make a serious criticism about it.
BEM takes place in Libra City, which is divided into Upper City (the city of the rich) and Outside (the city of the poor) through a bridge. The three protagonists meet on this bridge every night (and surprisingly no one ever discovers them there). The three Youkai Ningen save humans from different monsters, hoping that in the future they will become humans themselves. The plot doesn't sound THAT bad, right? Generic, yes, but bearable. However, it's worse than that.
As I said earlier, the trio thinks they will become humans after saving other humans. Although the idea sounds stupid, Bem himself admits it doesn't have much foundation, so I'll ignore it. I'm more interested in talking about that desire to become humans they have, which hardly develops in the course of the whole series. We know they want to be human, yes, but nothing deeper than that. Personal reasons? No. Moment when they decided to pursue that dream? Nah. Other ideas to get it? To hell with that, they want to be human and that's it. Obviously, the anime ended without resolving this issue and instead gave us a fight against a giant that came out of nowhere. Finally, as the cherry on top, we have an open ending that leaves many questions unanswered.
We also have the individual stories of each character, which don't lead to anything interesting, to be honest.

I will focus on the three Youkai Ningen, Sonia Summers, Roddy and Daryl, and then I will make a brief summary of what I think of the rest I haven't named. First we have Bem, the leader of the trio and the poorest, since he has almost no personality. Unlike Bela and Belo, Bem's life revolves around becoming human. However, because he isn't the only one who fights against the monsters that lurk the city, he practically doesn't possess any trait that individualizes him from his companions. Although we could consider his interactions with Sonia as a distinctive, Bela and Bem also have contact with humans (and, let me tell you, the interactions between Bem and Sonia aren't memoriable at all). On the other hand we have Bela, a Youkai Ningen who attends school to feel more human and interact with them. Believe me, every episode focused on her was a torment. Due to her naive nature, harsh attitude and avenger complex, it's difficult to get attached to her. And let's not talk about her relationship with Roddy, which was only a tease for 12 episodes. In the end, we have Belo, the youngest of the trio, who hasn't yet passed his edgy stage and makes comments abut humans's nature as deep as a pot of mayonnaise. Don't get me wrong, Belo is my favorite out of the three, but his structure as a character is just as poor as that of his partners. Belo doesn't want to become human as much as the other two, resigning himself that this will never happen. However, this doesn't justify his bitter personality or his choice in enviroments (he wanders on Outside, while Bela studies in Upper Town, but we never have a clue as to why the younger of the two chose to settle there). Considering that his personality doesn't develop into something different in the course of the entire series, there isn't much to talk about him (although it isn't as if the other characters had any development either). But hey! If you thought this sounded painful, I regret to tell you that these three static monsters were even better treated compared to the secondary characters: Sonia, Roddy and Daryl. The reason why I put these three together is because the three play the same role: being flat and one-dimensional characters that serve so that the three protagonists can rely on them. Because c'mon, Sonia's "goody two-shoes" personality, Roddy's crush, and Daryl's unreal revenge, only served to give Bem, Bela and Belo more material to participate with. That's why it's not surprising that they made Sonia adopt a little girl (with whom the writers didn't know what to do) at the last moment, that Roddy got set aside without a clossure of his own, and that Daryl got easily forgiven by Belo and then became the head of an organization that controls Libra City, even though he's 13 years old. I'm not even going to talk about the Mysterious Lady, since we'd barely met her in one of the last episodes and then she already portrait herself as the main villain. And not to mention Agent Joel Woods who literally had no reason to exist, the ridiculously racist Helmut Felt, the stagnant and generic Dr. Recycle, Bela's friends that on the last episode turned out to be bitches, or the cartoony villains, God. The only characters I would save would be Belo's three friends, because although they had no development either, at least they were kinda sympathetic.

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To be honest, this series can be entertaining if you like to sit down and not think much about what happens. However, if you are interested in well-developed characters and a promising plot, BEM will leave a bad taste in your mouth.