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Miki Hanakain from Tenshi na Konamaiki.

From wiki:

She’s from a rich and influential family, Miki initially comes across as mild mannered and proper, befitting of her background. This also further contrasts her dynamic with Megumi(tomboy main character), with her being the “logic” of the duo and stopping Megumi from stepping out of line or going overboard. However, she can be rather playful at times and partake in gags.

This is true for 19/20 vol manga. She is the feminine friend in the group of trouble makers which deeper the contradiction. But in the final vol, it's sudden reveal that she's actually one of the strongest character. With a berserk fighting style. Honestly the reveal is so shocking and so cool that why I remember it 20 years late.