(shinigami) ryomen / ichimen incidence and spirit pearl mechanisms
there's a relatively high incidence of (憑依; HYOUI) spirit possession among shinigami. it's not uncommon, to the point where there's a names for it: (両面; RYOMEN) , meaning two faced, and (一面; ICHIMEN) meaning single faced.
spitfire and kusuriuri are the only ryomen from the main cast
there's a lot of "incomplete" ryomen, so they're called smth different (ichimen) but also have a "true" magic induced transformation stage
spitfire's peach pit is an example of the type of spirit possession that occurs from a spirit pearl (霊真珠; REISHINJU) coalescing over time after absorbing cursed vengeful spirit residue (呪霊怨霊残存; JUREI (curse spirit) ONRYOU (vengeful ghost) ZANSON (the remainder of what survives)) from the atmosphere via osmosis. the (霊真珠) spirit pearl becomes enmeshed with the flesh organs in the body (in spitfire's case, beneath his sternum) and is inextricable and intrinsic.
in the manga someone (a normal non-shinigami human) will suggest removing it and he'll be like you want to lobotomize me...?
and kusuriuri will be like. pah! Amateur 💅
(a question implicitly raised throughout is, "are they shinigami or hitmen?")