Fukasaku

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.

Personality
Fukasaku, like his wife, is considered old and wise, and is greatly respected by all who know them well enough. In reality, he constantly bickers with Shima over small things when they should focus on the bigger picture, such as the fact that a battle will cause them to miss dinner. They are still close, as Shima is heartbroken after Fukasaku is killed by Pain.

It would seem that only Fukasaku can remain serious in even the worst situation, while Shima will constantly find a way to bicker with Fukasaku. She, similarly, does not have enough respect for the Great Toad Sage, calling him senile for constantly forgetting everything.

Shima and Fukasaku both seem to like Naruto. Fukasaku met Naruto for the first time when he brought the news of Jiraiya's death to Konoha. He found Naruto's nickname for Jiraiya ("Pervy Sage") to be both very fitting and very funny, but observed that Naruto, as he was, would not stand a chance against Pain, and offered to take Naruto away to learn Senjutsu at Myōbokuzan. After what was (for Naruto) a confusing introduction to sensing natural energy and drawing it in to blend Sage Chakra, Fukasaku (in light of Naruto's rapid progress) believed that Naruto would surpass Jiraiya in power. Naruto refurs to him as "Gramps" during their battle against Pain and the invasion of Konoha.

Hunt for Itachi arc
Jiraiya summoned Shima and Fukasaku during his fight with Pain to help him enter Sage Mode. In addition to sitting on his shoulders to provide him with natural energy, they gave him advice, and attacked Pain when necessary. Their combined efforts were enough to combat three of Pain's bodies on equal terms. Shima and Fukasaku then created a genjutsu that allowed them to kill the three bodies, but they were soon overwhelmed by the appearance of three more.

Knowing the battle was lost, Jiraiya had Shima take the Animal path he had captured back to Konoha. While she did that, Fukasaku accompanied him during his final confrontation with Pain. When he discovered Pain's secret, he carved a message into Fukasaku's back, and sent him away before dying of his injuries.

Senjutsu Training arc
Fukasaku informed Konoha and Naruto Uzumaki of Jiraiya's death, telling them what he could of Pain, and allowing them to photograph the message on his back. Knowing that Naruto wanted revenge, and that he was currently ill-prepared for a battle with Pain, Fukasaku took him to Myōbokuzan to teach him senjutsu. While Naruto practiced tuning in with nature, Fukasaku beat him (and his shadow clones) with a staff to keep him from taking in too much natural energy.

Naruto rapidly progressed through the training stages, earning praise from Fukasaku for so quickly catching up to Jiraiya. Once Naruto attained Sage Mode, he taught him Frog Kata. After Naruto mastered the fighting style, Fukasaku began the final step of trying to fuse with Naruto as he had done with Jiraiya, something that proved impossible because of the demon fox sealed within Naruto. Worried that all his training would go to waste, Naruto tried to find a way to achieve Sage Mode without Fukasaku.

Invasion of Pain arc
While Naruto continued trying to find an alternative, Fukasaku was informed of Kōsuke's death, indicating something was happening in Konoha. Shima, who was shopping near Konoha, also noticed that something was wrong in the village. She summoned Naruto, Fukasaku, Gamaken, Gamabunta, Gamakichi, and Gamahiro to Konoha's center moments after Pain destroyed the village. As Naruto engaged Pain, Shima and Fukasaku provided assistance, dealing with the Animal path's summons. They were amazed by his perfected Wind Release: Rasenshuriken, something he had not shown them during his training, but were again forced to deal with the Animal path.

Knowing that the Animal path must be gotten rid of, Shima coated the area in a dust cloud while Fukasaku threw Gamabunta at the path, giving Naruto an opportunity to finish it off. Pain was soon left with only one body, and Shima and Fukasaku tried once again to use a genjutsu to defeat it. Pain killed Fukasaku and knocks Shima away before they had a chance. Their bodies were later found and Shima, once healed, wept for Fukasaku. When Pain was defeated, and subsequently chose to revive everyone he had killed, Fukasaku was brought back to life, to Shima's shock and surprise. Later, Shima and Fukasaku were both seen together, standing atop Gamakichi's head and watching from a distance as the population of Konoha welcomed Naruto back.

Abilities
In battle, Shima and Fukasaku are extremely formidable. After Shima extends a serpent-like creature from her stomach to seek out the opponent, Fukasaku can quickly strike with his tongue. Both are capable of exerting powerful gusts of wind. Also, they seem to be very intelligent, as they quickly determined a weakness in the way Pain used his Rinnegan.

Senjutsu
Shima and Fukasaku are able to use senjutsu. Senjutsu is a specialized field of jutsu that allows the user to sense and then gather the natural energy around a person and draw it inside them, then blending it with their own chakra to create "Sage Chakra". This natural energy-empowered state grants the practitioner a high amount of a type of chakra which is stronger than other types of chakra. Other benefits include an unnaturally high level of raw strength, incredible speed, and huge resistance to physical attacks.

Genjutsu Skills
Shima and Fukasaku are capable of using a very powerful genjutsu called Demonic Illusion: Toad Confrontation Singing. This ability canceled out Jiraiya's lack of aptitude for genjutsu, making their combination much stronger. However, this genjutsu takes time. Also, in order to use this technique, both Shima and Fukasaku must adjust their voices and harmonize with each other (something neither of them seems happy about doing). When finally applied, it attacks the opponent's sense of hearing, and will completely paralyze the target(s) body. It gives Shima and Fukasaku a sore throat afterwards, meaning it can only be used once.

Trivia

 * Fukasaku is one of the characters in Naruto to experience death and be brought back to life. The others to have experienced this are Gaara, Kakashi Hatake, Shizune, and the many victims of Pain's wrath on Konoha.
 * Fukasaku's name comes from the legendary film-maker Kinji Fukasaku, best known for the WWII film Tora! Tora! Tora! and the ground-breaking yakuza film . The latter film starred the famous actor Bunta Sugawara, who gave Gamabunta his name.