All of them did. He was the one pushing everything forward in pretty much every route. The heroines did their own things of course, but Riki had to deal with it all too. Every bit of drama was something that both Riki and the character of focus worked through, whether it was Kud’s family situation or Kengo’s opening up.
Someone else’s backstory doesn’t have to effect the main character. The back stories matter little anyway outside of Haruka’s and Komari’s route, and bits of Kud’s route. You put a big emphasis on backstory… but, for example, the backstory in Haruka’s route is like… a blip in the route. It’s like 1/10 of the VN content iirc. Kud’s backstory is a paragraph about her grandparents and a couple sentences about her mom. They are tiny tiny aspects that, although shape a part of what made the heroine, have little affect on the characters in present times.