I have a little theory about Ren from this game! Essentially, I think he grows up to be Ray in Magical Melody!
So in River King: Mystic Valley, Ren is a character who is searching for the River King, and he's extremely devoted to fishing. He prefers to be alone and is pretty grumpy. Now I don't know what his age is, but I would guess he's a teen, given the context of the game. This is what he looks like:
Then there's
Ray from Magical Melody. While they don't have the exact same name, the names are pretty similar, and HM/Natsume often alters names (Gray to Blue, Mary to Marie, etc), so I don't think it's a big deal. But anyway, this is Ray:
Ray is described as a "
shy and solitary wandering fisherman. He is happy to talk to you about fishing, but will say little about himself until you befriend him. He has a weekly program on the Variety Channel, and gives information about where the
King Fish can be located.
Not much is known about his past."
It seems to me that Ren grew up to become a wandering fisherman who gives tips on how to catch the River King! And along the way he matured into a less grumpy guy, but still prefers going solo. Or at least, that's my headcanon anyway.