Pretty sure yajima cleared up the ambiguity in his interview.
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Serena told Ash that she's going to improve herself, and then come back to him
Here’s what she said in japaneze :
- anata wa watashi no mokuhyou yo. Tsugi ni aetara motto miryokuteki na josei ni naru kara *wink*
Notice that the word she used is
mokuhyou which means purpose, objective, aim, goal, intention, policy, target etc. It’s something you want to achieve. Serena is saying that ash is her prize and she wants to get him.
The very next line translates to “next time we meet i’ll be a charming woman” and that’s pretty self-explanatory : Ash is her target and she wants to impress him/be with him.
Because it’s tied to her happy ending. By saying that you want her to move on you’re wishing misfortune on her. Imagine taking away someone’s happiness only to satisfy your ego.