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Ash and Serena were childhood friends. She first met Ash at Professor Oak's Summer Camp. One day, while at the camp, she got lost in a nearby forest and was startled by a Poliwag, falling over and hurting her knee. Then, she met Ash Ketchum, who was looking for the same Poliwag and spotted Serena. Noticing her knee injury, he pulled out a handkerchief and wrapped it around her wounded knee, saying that it was a good-luck charm and that the pain would go away. However, when Serena's knee continued to hurt, Ash told her "Don't give up till it's over" and helped her up before escorting her out of the forest. Afterwards, she returned to Kalos, bringing along the handkerchief and the memory of her meeting with Ash.
It is revealed that Serena has kept Ash's handkerchief and has been meaning to return it to him ever since they had first met as children. While watching TV one afternoon, she saw a boy dangling from the top of a tower in an attempt to save a Garchomp, and realized she recognized him from somewhere. Upon realizing it was Ash, she set out to find him, following her dreams instead of her mother's wishes for her to become a Rhyhorn Racer.
This ship is one of the most popular ships in the Pokemon Anime. It's also one of the most popular ships between Ash and a female companion. The popularity of the shipping began during the season 1 of the XY series due to Serena being revealed to have known Ash in her childhood, and had a canonical crush on him. Additionally, in the season finale of Pokemon XYZ, before leaving for her flight to Hoenn, Serena went up the escalator and kissed Ash on the lips and leaves down the escalator as she goes to her plane and leaves for Hoenn. This made fans very excited and fanboying and fangirling that Serena managed to kiss Ash. On Twitter, Tetsuo Yajima, the director of Pokemon XYZ, confirmed that while they couldn't show the kiss so that the episode could air on TV, it definitely happened. He also said that Ash and Serena possibly become a couple in the future of the Pokemon Anime. It rivals with PokéShipping (Ash x Misty), AdvanceShipping (Ash x May), PearlShipping (Ash x Dawn), NegaiShipping (Ash x Iris), AlohaShipping (Ash x Mallow), AureliaShipping (Ash x Lillie), and BlueAngelShipping (Ash x Lana).