Fukasaku (フカサク) is the husband of Shima and an elder toad from Myōbokuzan. As the heirs of the Great Toad Sage, he and his wife are revered as the Two Great Sage Toads. Together with Shima, he taught Jiraiya how to use Senjutsu and later he alone taught Naruto the same. He has a leading position among the toads, being called Head (頭, Kashira).

Source: http://naruto.wikia.com/wiki/Fukasaku