Okay, I'm trying to clear the cobwebs a bit here. Your setting of DB_IsPlayer() looks fine. When I added NPCs to the party in my project, the only thing I did differently was to set the owning character at the same time when calling CharacterMakePlayer (see wiki link for notes). Example:

CharacterMakePlayer(_NPC, _Char);

Where '_Char' is the player who owns control of the NPC. Note that the call to CharacterAddToParty is deprecated and not supported in DOS2.

I am hoping what happened is that your NPC was never associated with a particular user (human player), and therefore the game did not recognize any survivors when your main character died. I would modify your CharacterMakePlayer() to look like my example and see if that helps.

DOS2 Mods: Happily Emmie After and The Noisy Crypt

Steam Workshop
Nexus Mods