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 is surprisingly gets along with Riki.
His name, Saiki Kumagoro, is a pun on the word Psychic (サイキック Saikikku), just like the rest of the Saiki family.
↑Japanese term for a character development process that describes a person who is initially cold and even hostile towards another person before gradually showing a warmer side over time.