1) If they don't remember witnessing the death, either due to amnesia or age, the spirit can still show up to them.
1a) However if Diavolo knows a certain dead person that Doppio doesn't, said spirit would only appear when Diavolo is in control of the body.
2) The spirits are bound to the character and cannot go away from them or go hang out with other people without said character being present.
3) It's absolutely possible to have the spirits of the nameless people they've killed show up. They'd obviously all be silent. Again, it's up to the player to decide which ghosts show up, so even if many characters would technically qualify for having a ghost army surrounding them, they don't necessarily have to go that way with this event.
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1a) However if Diavolo knows a certain dead person that Doppio doesn't, said spirit would only appear when Diavolo is in control of the body.
2) The spirits are bound to the character and cannot go away from them or go hang out with other people without said character being present.
3) It's absolutely possible to have the spirits of the nameless people they've killed show up. They'd obviously all be silent. Again, it's up to the player to decide which ghosts show up, so even if many characters would technically qualify for having a ghost army surrounding them, they don't necessarily have to go that way with this event.