You mention two things here that make very much sense. First the koi carp hobby/breeding. I can so see Iwaki the older do that. It would fit him perfectly. (I have friends who breed koi, and even keep the sick ones in their bathtub in the winter. They are insane!) It seems like a very dignified fish to me, if that makes sense.

(And if Iwaki the older was really a Yakuza boss... OMG, now I bunnied myself wildly!... there would be a gang war and the opposing gang, the Asano family, would end up inviting the Iwakis to a pretend peace dinner and the main course would be Iwaki's prize kois, stolen behind his back.) *stabs bunny until ded and silent* Ah, well. Now I have to write Iwaki/Katou AU. Bugger.

Masahiko... yes. He could very well have wanted to be a painter, he is sensitive and thoughtful - but has, as you say, suppressed it. Somehow it has to be something to do with visual arts... can't precisely say why.

"...when Katou tells the brothers how alike they are, and they practically say in chorus, "That's not true!" and "We're nothing alike!"

Ahaha, yes! It was perfect. Also, the scene when Masahiko visits, and Katou realises that he just needs to treat Masahiko like he treated Iwaki (without the sex, obviously) when they first met, and he finds Masahiko extremely cute then.

I have a bit of a crush on Iwaki's dad, to be honest. He's one of the good guys, and stern men with a heart of gold... well...