Both shows are psychological thrillers where the bad guys want to change a "fallen" Japan by singling out their "chosen ones." Both are more centered upon the mysterious plot than any romantic themes. Part terrorism, part friendship; the stories give a little bit of the same feel. Fallen Angel is shorter though and a little bit less in depth/drawn out.

They are both about traumatized young boys finding brothers to protect and get them through. In RH+, the brothers are adopted and vampires; in Fallen Angel, they are actual kin. But still, the bromance feeling is the same. Both shows are mainly centered around the boys and have supernatural/psychological thriller themes.