Kikuno Yamada is the mother of Kirio and Aoi Yamada, and one of the executive assistants of Shizuka Takanashi. Like her children, her real last name is unknown, but upon finding out that Aoi was calling herself Yamada, she also began addressing her as Yamada.
Kikuno's defining trait is that she overthinks things, her mind races ahead in any conversation to the point where she can't express herself verbally at all, which leaves her with a gloomy expression. Her husband had to create a manual ("Secret Mom Manual") for their children to understand her, although Aoi never read this manual. She is also adept at espionage, assassination and performs as a body double for Shizuka at times.
She is defined by her daughter as "kind" person, which hinted that Aoi never really ran away from home because she has unthoughtful parents. (Kirio also defined her mom as "emotionless")
Kikuno has not been able to work properly over time since Aoi was gone, she tends to mess things up and break things such as her computer.
It is hinted that she was more talkative before her husband died. In ep 7 of season 3, Aoi told her story to the Takanashi's wherein she said that she has a happy life but when her father died, her mom changed and after that she stopped talking. As a result, Aoi was forced to study all the time; however, this is just a one-sided statement by her daughter. As mentioned, her mother never really wanted her to hate studying and was practically not forcing her to study, she just couldn't think of any way to cheer her up and instead gave Aoi study aid books that may help her cope, but Aoi being a child and not bothering to read the manual left by her father, never took it that way. She simply thought that her mother was forcing her to study.
Kirio understands her more than Aoi does. He tries to understand what Kikuno is thinking through her expressions and the look in her eyes.
It is revealed that her husband used ventriloquism for her children to be able to converse with them when they were younger. It is also revealed that when Aoi failed the entrance exam, Kikuno wanted to comfort her daughter but she didn't know what to say and instead, she tried to find study-aid books that may suit her.