The relation between Kota and Onsen is very fascinating to me .It does not have a fixed correct answer to it and left up to interpretation of the viewer(I view them as soulmates).Their existence came from the same river and they could be brothers in that sense but thinking about it more clearer ,they are the same person split into two and now they want to be a whole again,self love indeed.
And i wonder if the colours of their scales have another meaning to it rather than it just being "one is blue because cold and the other is red because hot".