If Raizo is called Raizo of the Mist (Kiri no Raizo), could it be possible that Shiliew is also a ninja because he is Shiryu of the Rain (Ame no Shiryū)?
All Wano’s characters are named first by an alias, we have:
Kitsunebi no Kin´emon (Foxfire)
Yudachi no Kanjurou (Evening Rain)
Kiri no Raizo (Mist)
In addition, when Shiliew first killed his subordinates and apologized, according to his trivia on the wiki: the sarcastic apology Shiliew gave to the guards he cut down “Kiri-sute gomen” (斬捨て御免? literally meaning “authorization to cut and leave [the body of the victim]”) is an old Japanese expression for a samurai’s right to strike down an individual of lower class who compromised their honor. In modern days, the term is still used as an advance apology of sorts, with the subtle humor in offering what amounts to an unsympathetic apology.
So that itself could be a nod towards him being or having connections to a samurai of Wano. Now… is Ame no Shiryu (Rain) an exiled samurai of Wano Kingdom? Are we going to see him during Kaido´s Arc?
*Theory by sir_tejj