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Aoki (青木) is the fourth butler and financial manager of the Yotsuba Family. 
Appearance and Personality
Aoki is described as a shrewd and capable gentleman in the prime of his years, depicted as someone who looks more like a lawyer than a butler, all the while being a middle-aged man. 
As the financial manager, he is responsible for overseeing all of the assets related to the Yotsuba Family.  He has for over 10 years dealt with the underground economy and has guarded the Yotsuba treasury.