In this manga is the old main character appears? Or is it whole new different manga

I doubt he'll appear but who knows. I only read a couple of chapters. This is basically about Hirakawa and how things started with him teaming up with the parasytes. I guess you can say that his son is one of the main characters here as well. Pretty interesting. After I finished Parasyte Hirakawa was the person I was most interested in. He was a peculiar fellow.