There's a fire in his eyes. When he speaks of Seimei, in those rare moments spent just the two of them, it's then that Soubi is graced with a glimpse.
Like a flame on the wick does his slight body move, graceful and passionate. Ritsuka is sheer poetry- the flick of an ear, of a tail, those gapes and glares. Always flowing, the flames growing brighter as his will carries him throughout life.
The summers end, winters are fierce and though external Ritsuka is diminished his eyes aren't. The fire is eternal, something that distinguishes him from anyone else, as identifiable as a scar or tattoo.
Perhaps one day he'll allow Soubi to tame that fire. It's the piece of Ritsuka that Soubi first fell in love with, something he isn't willing to share. There's times he doubts even Ritsuka is conscious of it.
Soubi wants to protect that flame, he knows that above all. It is the sole meaning of his existence.