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New Serena will be a mature prim proper confident girl who behave as all the guys dream idol except when it comes to Ash, she will show more extreme crazy side to her personality which will be often use as a joke. She will always act flirty around Ash like Melody/Miette but goes Yandere when other girls makes move on him. The season for this change is the kiss at the end of XY&Z series, because she already kissed Ash so she will feel no shyness and hesitation around Ash causing her to be openly affectionbate toward him which including hugging. There also a downside of all the guys that crushing on Serena perceive Ash as their enemy, there will be some rival who has rivalry with Ash due to their crush on Serena. Ash on the otherhand is scare to make Serena angry because she no longer hesitate to smack him if he does anything dumb.
Kind of like Mami Kirara, speaking of which Yuyama actually work on Purin anime. Considering its well reception , I guess Yuyama had the last laugh.