Saiki Kumagoro

 Kumagoro Saiki (斉木 熊五郎, さいき くまごろう lit. Saiki Kumagoro) is the husband of Saiki Kumi, the father of Kurumi, the father-in-law of Saiki Kuniharu and grandfather of Saiki Kusuo and Saiki Kusuke.

Background
Kumagoro is a grumpy old man who hates Kusuo's father Kuniharu and loves his daughter Kurumi. He is actually a tsundere who truly wants to dote on his grandchildren. He surprisingly gets along with Riki.

Trivia

 * His name,, is a pun on the word Psychic (サイキック Saikikku), just like the rest of the Saiki family.
 * Kumi and Kumagoro's Japanese voice actors, Rie Tanaka and Koichi Yamadera respectively, were actually married at the time of their debut episode's production, but the two divorced in 2018.