Saiki Kusuo[]
His grandson, Kumago portrays a dotting grandfather to him and often does what he can to spend more time with to get to know him. Unknown to him, Kusuo used his powers to circumvent this each time, not interested in spending more time than he needed with him. However, he was touched when Kumago tried to protect him on a spiraling Ferris wheel. Afterward, Kusuo and his parents decided to show Kumago and his wife some of Kusuo's powers, amazing them. The two agreed to keep it secret, while Kumagoro thought it was a way to know his grandson better. Kumagoro was pleased when it seemed like Kusuo was worried when Kusuke made him into a robot.
Saiki Kusuke[]
Like Kusuo, Kumagoro is happy to see Kusuke in order to dot on him. He acknowledges his grandson's super intellect, noting that it explained some aspects of Kusuo. He almost died of happiness when Kusuke decided to move in with him, but did get upset with Kusuke for trying to use him and the rest of his neighbors to fight Kusuo and causing Kumi to age faster.
Saiki Kuniharu[]
For reasons unknown, he dislikes his son-in-law and often calls him a pathetic parasite. Despite his happy marriage to his daughter and kind nature, Kumagoro does not hesitate to belittle him. When he went too far by excluding him, he was scolded by his wife and daughter. Thanks to Kusuo, Kumagoro gave him a new set of glasses to replace the ones he broke. As a result, both his wife and daughter were proud of him for his supposed apology and he was forced to go along with it.
Saiki Kurumi[]
Saiki Kumi[]
Teruhashi Kokomi[]
Kokomi met Kumagoro by chance when he lost his wife Kumi somewhere in the city. After dripping a call that his wife is lost, Kumagoro sat dejectedly in front of the telephone post when Kokomi talked to him. He thought she was an angel that saves him. Telling her the problem, she then lends him a hand and asked for the name of the house Kumagoro was looking for. After learning it was the 'Saiki' house, Kokomi got the clue that he was actually Kusuo's grandfather. She then tried to curry flavor the talk but it ended to her demise as Kumagoro was a doting grandfather. The meeting of the two worried Kusuo but he later stopped following the two because it was too unbearable. In the house, Kumi casts her vote for Mikoto as her granddaughter-in-law but Kumagoro wasn't keen to her idea as he wanted Kokomi for Kusuo.