>>52852544You really should just treat every series from BW onwards as its own continuity with some fanservice nods to the overall anime. Ash's personality drastically changes with every season too.
Serena is the endgame for XY Ash, who's a distinct character from OS-DP Ash, BW Ash, SM Ash and JN Ash. JN Ash has no endgame as he's just an amalgamation of all previous incarnations.