May be the single most redundant, outdated and random thing anyone's ever said but here we go lol.  
  
I always found it strange going through Mooshu that while Warlord Katsumori is the focus of the War Quests and is very pointedly a figure who is upfront in all the dialogue and discussion- with the War Oni being a surprise reveal- the same does not go for the Plague and Death quests. As far as I'm aware, neither Ideyoshi or Kagemoosha is mentioned either by name or even referred to by another character, with the work of everything only being referred to as being the Plague Oni or Death Oni. It could be assumed based on the Mooshu History books that they are each the head of the 3 clans who used to control Mooshu and remain today, but that's mainly speculation since again, literally nothing is ever said about these two. Their monstrology entries are just (bad) humor, which kind of pisses me off that Kingsisle had the opportunity to provide us fun lore snippets with Monstrology and most of the time just spends it on pretty eh jokes. Ideyoshi is either just the Plague Oni in a different form if we go by the Plague Oni's monstrology entry, as it said the Plague Oni transformed into him, though this can be taken as possession too. Alternatively Ideyoshi could have been charged with protecting the River Temple before his possession, I say this because he resides in the River Temple to begin with, and secondarily while the Plague Oni itself is Balance Ideyoshi is Ice, one of the two schools (the other being Storm) traditionally associated with water in Wiz. It's also possible that Ideyoshi is an actual character who once existed and the Plague Oni is using his form as a disguise instead. Kagemoosha is a little less speculative, the Death Oni's entry states he's the reincarnation of the evil Kagemoosha, so it's probably true that Kagemoosha just IS the Death Oni, all the characters simply refer to the one who took the Spirit Candles into the Spirit world as Oni no Death despite the fact that's clearly Kagemoosha, and Kagemoosha needing the candles to summon a portal to the spirit world implies he's still living, so idk. The specific use of the term "reincarnates" implies Kagemoosha is both living and also that the Death Oni is literally just Kagemoosha in a different form, though given everyone in dialogue refers to Kagemoosha as Oni no Death it's equally possible this could just be the Death Oni already incarnated into a person and Kagemoosha dying just returns it to its original form. So all in all for the Oni, the War Oni is something Katsumori turned into when he started... well... war. Ideyoshi is either just the Plague Oni taking on a different form or disguise or is some unknown pre-existing figure who turned into the Plague Oni or was possessed by him. Oni no Death is probably literally the same person as Kagemoosha and they are just in different forms and as long as Kagemoosha has existed he's just been the Death Oni. The Jade Oni is the only one who confirmedly is possessing someone and has no other identity, and you could even speculate that given the Jade Oni has the Dragonspyre Spiral Key that Malistare literally cursed the Emperor with the Jade Oni directly.

Do Ideyoshi and Kagemoosha have literally ANY background?