The difference between FB and HR on the character exploration level is that the characters in FB have a much more literal explanation of them whereas the characters in HR are more vague, purposely, so that whoever is watching can see themseleves in their place. That however doesn't make Rakka and Reki any less of characters but I think makes their characters even stronger than if they were looked into more and easy to connect with than most other characters in other shows.
I think ABe's intent for the series was to create a show which people can see themselves experiencing these things. By making parts intentionaly vague and a heavy use of symbolism he manages to make a series that is universal to the entire world. No matter where you are in the world it's human nature to go through the emotions shown in the series at some point in your life. It's one series where the language barrier doesn't make a difference.