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SERENA ISNT FUCKING REAL
JOURNEYS ESTABLISHED THAT ASH NEVER WENT TO SUMMER CAMP OR SAW THE "Girl in the straw hat".
Now, since we see Serena's origin is from a summer camp that Ash, in fact, never went to, we can assume that Serena was manifested from Ash reflecting on his lost dreams on the plane ride over to Kalos.
Since he had just suffered a humiliating defeat at the hands of Cameron, this is only natural.
Notice Serena's motif through the series, "DreamDream", she's a character who chases her dreams.
She inspires Ash to try once more at becoming a League champion.
But we all know how that goes.
The only remaining question is, what "Girl in the straw hat" was Ash referring to. I've been trying to scour old episodes for any COTD that could be wearing a straw hat, it may give us a clue as to Serena's whereabouts or continued existence (If any)