Aoki is the fourth butler and financial manager for the Yotsuba family.

He 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.

While extremely respectful when talking to Miyuki, he views her elder brother Tatsuya as nothing but a Yotsuba servant, hence his acts of blatantly ignoring or being rude when addressing him, much to Miyuki's anger.