One of the Two Great Sage Toads, he was also taught Naruto how to use Sage Mode
Itachi Pursuit Arc
Fukasaku and Shima activating Sage Mode with Jiraiya
Fukasaku is first introduced during the Itachi Pursuit Arc. Alongside his wife (Shima), Fukasaku was summoned by Jiraiya during his brutal battle against Pain in order to aid him in entering Sage Mode. Fukusaku and Shima both aided Jiraiya by providing him with the natural energy required to fuel Sage Mode, not to mention conducting battle strategies against the mighty Pain. At first it seemed as though with the aid of the toads, Jiraiya was indeed an equal match for Pain as they simultaneously fought well against three of Pain's Six Paths. Soon, the toad duo unleashed a genjutsu that helped them kill the three bodies of Pain. Unfortunately however, even with the help of Fukasaku and Shima, Jiraiya could no longer hold his own when three other bodies appeared and did battle with him. It was clear that Jiraiya had no chance of being the battle's victor, this prompted him to have Shima take Pain's Animal Path to Konoha. And as the Animal Path was taken to the Hidden Leaf Village (Konoha), Fukasaku and Jiraiya continued to do battle with Pain. But it was becoming far too obvious now that Jiraiya was going to be killed. Moments after discovering Pain's secret, Jiraiya carved an encrypted message on Fukasaku's back before dying.
Invasion of Pain Arc
The Great Sage Toad had the unfortunate burden of informing Naruto and Konoha of Jiraiya's death at the hands of Pain. He also informed them of what he knew about Pain and his abilities. The main objective he had other than informing Jiraiya's comrades of the shinobi's death was show them the message that Jiraiya had left on his back. After realizing that Naruto intended to exact revenge by defeating Pain, Fukasaku decided to take Naruto to Mount Myoboku for training. With the intention of teaching Naruto senjutsu, Fukasaku had him attempt to harmonize with nature while he in turn beat Naruto and his Shadow Clones with a wooden staff to stop him from absorbing too much natural energy. With Fukasaku's guidance, Naruto trained well and once he learned Sage Mode, he was taught the deadly Frog Kata fighting style. After Naruto had mastered Frog Kata, Fukasaku attempted to fuse with him just like he did with Jiraiya during their battle with Pain however, Fukasaku discovered that it was impossible to fuse with Naruto due to the Nine Tailed Demon Fox.
Fukasaku, Naruto and Gamabunta arrive to battle Pain
Upset, Fukasaku stated that all of Naruto's intense training had completely gone to waste. Naruto on the other hand did not concede and attempted to find another way to enter Sage mode without having to fuse with the toad. Meanwhile Naruto struggled to find another method of achieving Sage Mode, Fukasaku was sadly told of Kosuke's death, implying that something strange was taking place in Konohagakure. Fukasaku's wife Shima was near Konoha had also taken notice of something odd. Soon Naruto had successfully achieved Sage Mode by using his Shadow Clones to absorb the natural energy needed for Sage Mode. Soon after Pain had destroyed most of Konoha with his Shinra Tensei technique, Fukasaku, Naruto, Gamabunta, Gamaken, Gamahiro, and Gamakichi were all summoned to the center of the battlegrounds to fight Pain. In the meantime as Naruto fought Pain, Fukasaku and his wife handled with the summons of the villain's new Animal Path. Both Shima and Fukasaku were surprised as they witnessed Naruto use a perfected Futon: Rasenshuriken as they dealt with the Animal Path. The Animal Path began to prove to be quite troublesome, forcing Shima to coat the area with dust as Fukasaku threw Gamabunta at the path, granting Naruto the opportunity to destroy it (the Animal Path). With only one body left, Pain appeared to be on the brink of defeat. And just as they did with Jiraiya, Fukasaku and Shima tried to use genjutsu to defeat Pain's Deva Path. Unfortunately, the Deva Path killed Fukasaku and knocked Shima away before they could create the genjutsu.
Later when their bodies were found, Shima was healed only to be horrified at the realization that her husband was dead. She mourned his death but she was soon relieved when Pain was defeated he decided to revive everyone he had killed (except for Jiraiya) which brought Fukasaku back to the land of the living. Both Fukasaku and Shima were later seen with Gamakichi watching Naruto being welcomed back by Konoha's villagers.
Confining the Jinchuriki Arc
While he did not have major involvement during this event, Fukasaku did summon Naruto from Ramen Ichiraku to Mount Myoboku once again at the order of the Great Toad Sage. Fukasaku is present during the Great Toad Sage's conversation with Naruto. Fukasaku kicked Gerotora to help Naruto when the shinobi had difficulties swallowing the toad. Afterwards, Naruto was returned to his location prior to the summoning.
Being of old age, Fukasaku is considered to be quite wise. He is respected by those who know of him, particularly Jiraiya. Despite this wisdom, he often childishly argues with his wife Shima over the most ridiculous of topics such as the possibility of missing dinner when they should focus on an important battle. They have these fights even when their lives are on the line which at times brings their so called wisdom to question. Even though they constantly bicker, Fukasaku and Shima do truly love each other as evidenced when Shima is grief-stricken over Fukasaku's death at the hands of Pain's Deva Path. Although Fukasaku does appear to be the more serious of the two as he attempts to focus on the most important thing but unfortunately his wife usually finds something to bicker about. They also appear to lack a degree of respect for each other, Shima often calls Fukasaku senile due to his apparent forgetfulness. Aside from his relationship with his wife, Fukasaku seems to like Naruto. He also found that Naruto's nickname for Jiraiya "Pervy Sage" was both appropriate and humorous. However, he also took note that Naruto is a little reckless after seeing how anxious the boy was to fight Pain even when he was hardly prepared. Fukasaku also noted that Naruto drew comparisons to Jiraiya and after witnessing the boy's impressive progression in training claimed that Naruto would one day surpass even his former teacher Jiraiya in power. Fukasaku appeared to have had a good relationship with Jiraiya and considered him a friend. He was genuinely saddened at his pupil's death. He also appears to have some sort of relationship with the Hidden Leaf Village.
Powers & Abilities
Like his wife Shima, Fukasaku is quite the opponent in battle and should not be taken lightly. He is known for being a capable combatant and can even strike at alarming speeds with his tongue. He has also shown the ability to generate strong gusts of wind and he is also remarkably intelligent as evidenced when he discovered Pain's weakness simply by observing the manner in which Pain used the dojutsu known as the Rinnegan. This is a display of good observational intelligence. Fukasaku is also quite strong for his size. Fukasaku is very skilled at using senjutsu and genjutsu:
Senjutsu: A capable user of senjutsu, Fukasaku can sense and gather natural energy around himself and draw it inside him. He can then mix the natural energy with his chakra to create what is known as senjutsu chakra. In this state of power, Fukasaku has access to large amounts of senjutsu chakra, a type of chakra that is more powerful than the other types of chakra. This grants him great physical strength, speed and immense durability. One of his strength feats is throwing even Gamabunta a long distance, implying that he is much stronger than he looks when using senjutsu. Fukasaku is a master of senjutsu, being able to flawlessly gather natural energy.
Mugen: Gama Rinsho
Genjutsu: Fukasaku is a capable genjutsu user and can use a sound-based genjutsu called Magen: Gama Rinsho. This genjutsu was very useful when Fukasaku teamed up with Jiraiya since the shinobi is not particularly skilled at the art of genjutsu. However, a disadvantage is that this genjutsu is time consuming. This technique also requires Fukasaku to harmonize his voice with his wife Shima's voice. The technique attacks the target's sense of hearing and then proceeds to paralyze the victim's body. However, this technique cannot be used repeatedly as after using it once it gives Fukasaku and Shima sore throats. Fukasaku is one of the most skilled genjutsu users introduced in the Naruto series.