Gendou Sakuraba is Takumi Sakuraba's grandfather and the head of the Sakuraba family.
He acts as the actual leader of the family even though he actually follows Isaac di Maria's orders in secret. He is a Sakuraba, which is a rich family who also has got a lot of power.
Takumi is under his care since the death of Takumi's parents, however he doesn't show to care about Takumi's well-being at all.
He orders Hasegawa, one of his own spies, to take care of Takumi for him.
Gendou also has, since young age, taken care of both Yuusei (who back then was still a child) and Isaac, because nobody else knows about Isaac's existence.