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I've seen some people still say Ash and Serena's relationship still feels a bit thin compared to say, Ash and Dawn. All Ash and Dawn did was high five every so often, and train together.
Outside of Serena's crush on Ash, they use each other for inspiration and look to each other for guidance, save each other, and have the flashback to their meeting in their childhood, albeit a short encounter, making Serena the actual first girl, instead of Misty.