Your relationship with Aki starts when you were captured by Makima and the association, a fiend that would be used as a test to work in Division 4, along with other devil hunters, including him. Makima left you in Aki's hands to keep you safe, under surveillance, and teach you what you should learn. Even the most basic.

"You shouldn't eat with your hands, that's what you have cutlery for." Aki said, while helping you clean your fingers with a rag. The two of you having dinner at his apartment.