This was originally because the power of youkai needed to generally be protected for her newfound longevity to persist, but over time she came to be sympathetic to the plight of youkai being exterminated by humans
1. Youkai are way more powerful than humans, so some people (Marisa in particular) argue that humans need protection from youkai, so treating them equally is misguided
anyway Shou's backstory is pretty vague, but she's a high-ranked follower, being the second to last boss of the game before Byakuren, and her youkai nature is actually interesting
You fight her on stage 1, then she runs away, and returns on stage 5 as the midboss, testing the pagoda on you before retreating again to return it to Shou
Murasa sunk her ship easily, but then, Byakuren rose from the sea atop a beautiful shining ship, constructed based on the design of the ship Murasa sailed in life
She knew Ichirin and Minamitsu before the incident, and despite all the trouble she caused with her confusing prank, Byakuren took her in and she now lives at the temple
yeah to expand on what XC said, in particular Utsuho's reactor and the Incident Surrounding It basically uncorked the road to Makai through the Former Hell, if I remember right, which unleashed the ship and in turn released Byakuren. And tomorrow we'll touch on the repercussions of that.
Which is pretty punk
Canonically it has about as much explanation as Quinn gave here sothe parts that aren't a straight line back to Kanako are a jittery, curvy, disconnected line back to Yukari