Major spoilers to this game.
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1-Butterfly was killed twice. The first time in ''Happiness 1'' but he was resurected when the ''Happiness MC'' rebuild the old timelines at the end of that game.
He was killed a second time in ''Milker'' and instead of his soul passing away peacefully through ''Djara village'' he was chosen by the ''system'' to recieve punishement for his deeds so in other words this game ''Reeva'' takes place after the events of ''Milker''.
At the end of this game ''Reeva'' butterfly is going back to the Null's world to save Reeva and at the end of ''Devotion'' most of the major character including both ''Happiness MC'' and ''Goodbye time traveler MC'' are going to the Null's world to stop Destiny.
Basically every major character in the series including ''Butterfly'' ,''The Happiness MC '' and ''Goodbye time travler MC'' is going to the Null's world at the same time.
2- Reeva is the name of ''the kingdom of Reeva'' , the one you belong to in the game ''Existence'' and this kingdom is named after '' Princess Reeva'' the one that allegedly led the rebellion against the empire way way back in time.
If you play ''Existence'' and talk to an NPC named ''Ray'' in the town of ''Veradis'' you can ask him to tell you the story of ''Princess Reeva''.
The white haired girl Reeva in this game and ''Princesss Reeva'' from the game ''Existence'' are the same person.
She's the same girl .