Tobirama Senju

The is the younger brother to the First Hokage and Tsunade's great-uncle. As the brother of the First, the Second Hokage was a member of the Senju Clan of the forest. He is also the former teacher and team leader of the disbanded Team Senju. He is greatly known for his abilities with Suiton Ninjutsu being able to use high level water jutsu with no water source nearby. According to the third databook, his name is Senju Tobirama.

Background
The Second Hokage is the brother of the First Hokage Hashirama Senju, and part of the Senju Clan. 80 years prior to the current era, the world was in a time of war and the ninja clans were nothing more than assassins for hire for the various feudal lords. Because of his brother, the Senju Clan was constantly being hired for wars and in turn the enemy would then hire the Uchiha Clan. However the Senju would soon found it meaningless to continue fighting and offered the Uchiha Clan a truce, something Madara Uchiha was not very happy about, but did agree.

Eventually, the allied Senju Clan and Uchiha Clan would make a pact with the feudal lord of the Land of Fire and the Second Hokage would together with his brother and Madara Uchiha foster the village Konohagakure, and helped create the "one village per country" rule that would eventually end most of the wars between the feudal lords but their armies were still maintained in case of the four out of the five great shinobi countries had the biggest armies were still maintained by the countries at current level after the first ninja war. However there were still internal struggles and after the fight between Madara and Hashirama, the villagers soon began suspecting the Uchiha Clan of treason and plotting against the village, but Hashirama taking the Uchiha Clan's word, chose to do nothing, something unknown to his brother, who was not very fond of his choice and the Second suspected treason.

During this time would the Second Hokage also train a three person team of his own, called Team Senju consisting of Sarutobi, the future Third Hokage, and Homura Mitokado and Koharu Utatane, the two future elder advisers of the village.

However the war still waged on, and unsure of the title of Hokage, would Hashirama appoint his brother as the Second Hokage (Nidaime Hokage) and later the Second would also appoint his student, Sarutobi to be the Third Hokage (Sandaime Hokage).

After becoming the Hokage, the Second would give the Uchiha Clan a special position as a sign of trust. He gave them the leadership of the Konoha Military Police, however this was just a cover up, and its true purpose was just to distance the Uchiha from the government of the village, and place the clan under a tight surveillance, with the ANBU and Root.

Like his brother, the Second would eventually too die in the wars, and pass on the title of Hokage to his student Sarutobi.

Abilities
He was hailed for being the greatest master of water jutsu, being able to create water from nothing, allowing him to use any Suiton jutsu without requiring a source. However it could be speculated that he is just gathering the water vapor in the air to gather large amounts of water, for his all powerful jutsu.

For his Water Release: Water Dragon Blast Technique that usually requires the largest amount of Hand Seals ever seen in the Naruto Universe, 44 Hand Seals in total, he is only seen using two Hand Seals to activate and use it, a testament to skill.

Also, during his fight against Sarutobi, in the anime only, he displays great abilities in manipulating water, such as using it for projectiles, or tentacles to drag the opponent down in the water.

In an anime filler arc, it was said that he wielded the Sword of the Thunder God (雷神の剣 Raijin no Ken), a blade with electrical abilities.

Destruction of Konoha arc
He is later revived from the dead by Orochimaru using Impure World Resurrection to fight the Third Hokage during the Destruction of Konoha arc. His soul is eventually sealed into the stomach of the Death God, along with his brother and Orochimaru's arms.

Anime Filer Arcs
During the anime's Tea Country filler arc, it was revealed that the Second possessed a valuable retractable energy sword called Sword of the Thunder God Despite its appearance as an arc of lightning, the blade can be cracked and broken like a regular sword, as shown at the end of the arc.

Quotes

 * Its been a long time