Complex sentence: Both Heisuke and Naoko conclude that her spirit is possessing Monami's body; unable to explain what has happened, they decide to keep the matter a secret while Naoko lives as Monami from now on.
Split into simpler sentences: Both Heisuke and Naoko conclude that her spirit is possessing Monami's body. Unable to explain what has happened, they decide to keep the matter a secret while Monami lives as Naoko from now on.