True, that would be rude. OK, I might as well lay out my suspicions here, since although I think they've been mentioned before, it's always worth keeping them in mind.
I'm mainly focused on the fight against the Oni, and what happened in
this post and the ones after it. You know, when Sosuke caught fire and started screaming about the Sun.
So, first up, reincarnation is a thing in Rokugan. It's actually an empirically verified fact. More than that, even divine beings can die and reincarnate in the appropriate circumstances - Moto Naleesh is the reincarnation of the Kami Shinjo, for example. The only way for a mortal to escape the wheel of reincarnation is to live a properly virtuous life and then remain in Yomi as an Honoured Ancestor, and even then they might choose to reincarnate out of their own free will after a time.
(Well, you can also escape by having the Lying Darkness obliterate your soul, but that's not really a common thing.)
So that means that Hiruma Sosuke was previously incarnated as someone else, who might not even have been human - there are noted incidents of Naga reincarnating as humans, for example. It's difficult to pin down who he might have been, but in this life he is a Crab, and probably one with Hida blood given his massive size.
From his Gempukku, Sosuke has been receiving visions of a mysterious woman, who seems to know who he is and is waiting for some kind of development. When he fought the Oni, he started burning with brilliant fire, likened himself to the Sun and had a particular hatred of evil and anything which might threaten the world of men. There's only really one individual who I know of who also meets those criteria - Hida Yakamo, also known as Lord Sun before being cast out of the heavens and slain by the Jade Dragon who took his place.
So... yeah, that's my theory. Hiruma Sosuke is the reincarnation of Hida Yakamo, and thus the mortal reincarnation of the Sun.