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I come in peace.
White flag and all that.
ANYWAY, the thing about Misty and Serena is they're two different girls with different personalities.
Misty is stubborn and spends the first part of OG fighting with Ash over who is better. As time goes on she forgets her bike and genuinely enjoys his company, however, as tsundere girls tend to do, she refuses to admit to anyone or even herself that she's fallen for the same guy she's made fun of, so you get episodes where she'll latch on to other guys that she deems more mature and perfect for her, totally not that loser 10 year old, no way. She also karate chops a complete stranger for asking Ash on a date, then denies she has any problem with it.
Serena, sees Ash as a mature and nice guy, she's not ashamed of her feelings and wants to get closer to him, but she's shy so she can't just outright tell him. She gives little hints. Even if another girl shows signs of liking him, all she can do is silently scowl because she's not the type to get into verbal fights.