The whole connection between Jan and Rio comes solely from the fact that Rio had his unfortunate feud with Dan. I often read the conflict being about how Jan has to patch up some loose ends that involved Dan, Rio, and Gou. To the point where even Gou pleads Jan to stop Rio when he himself fails to stop him.
I'm not sure if I can agree that Ran and Retsu are bland or useless. I often felt like the two of them got a pretty bum rap (same with Ken). I feel like their roles in the story had more to do with being Jan's "older siblings," and get the immature newcomer to grow up. I always felt like those two taught Jan a lot more than any of the actual Kensei did. And I feel like their personalities fleshed out from their different backgrounds and different ideals. Ran being more focused and elegant. Retsu being more artistic and determined.
Ken often struck me as a character who's like the silly older brother who... I guess would kind of be a bad influence to the younger ones?
But he still means well. To me, he always seemed like the guy who felt like it was best to not grow up too fast. A complete opposite to the more mature and serious Ran and Retsu, and even Gou.
I feel like the roles of Gou and Ken in the story of Gekiranger are a lot more closer in style to the early Sixth Rangers, where they both bring with them a large scale epic story, dragging the main team into them. Gou has a pretty cool backstory with Retsu, Rio, Miki, Dan, and Xia Fu, which I think is a major bright spot of the show. And Ken opens the window that leads to further world building involving the Souzyutou, Brusa Ee, the Kenma, and the Kensei. Aside from that, I feel like they are more support for the main team, rather than the main characters of the plot (kinda like Blue Swallow from Jetman), and I think that's alright.~