Tobirama Senju

was the of Konohagakure. He hails from the Senju clan, who along with the Uchiha clan, founded the first shinobi village: Konoha. During his reign, Tobirama was accredited as the Hokage that developed the village's infrastructure and organisational system.

Background
Tobirama was a member of the Senju clan along with his brother Hashirama — who was the leader of the clan. Decades before the start of the series, Hashirama struck a peace treaty with the Uchiha and its then-leader Madara Uchiha and together the two clans created Konohagakure. When Hashirama, who was appointed the First Hokage, died, Tobirama was appointed as the Second Hokage. During his reign he was primarily responsible for establishing Konoha's organisational system. He established various organisations such as the Academy, the ANBU, the Chūnin Exams and the Konoha Military Police Force, the latter of which he created as a sign of trust towards the Uchiha who made up the majority of that organisation. However, according to Tobi, the Police Force was actually established to distance the Uchiha from the government of the village and place the clan under a tight surveillance by the ANBU. At some point during his tenure, he and the Second Raikage met to form a formal alliance between Kumogakure and Konohagakure; however, Kinkaku and Ginkaku, Kumo's Gold and Silver Brothers, attempted a coup d’état against the Second Raikage and left Tobirama near death during the fiasco.

He also trained a three-man team of his own, Team Tobirama, which consisted of Hiruzen Sarutobi, Homura Mitokado, and Koharu Utatane. During a mission in the First Shinobi World War, on which Team Tobirama performed with Danzō Shimura, Kagami Uchiha and Torifu Akimichi, they found themselves surrounded by twenty elite Cloud ninja, known as the Kinkaku Force. Tobirama then realised that the only way any of them could escape alive was for one of them to be a decoy and sacrifice themselves; therefore having faith that he had trained his students well, he chose to sacrifice himself after he selected his prized student Hiruzen for the position of Third Hokage. Homura and Koharu would later become Hiruzen's advisers. After giving the title of Hokage to Hiruzen, Tobirama went off to fight the enemies where he died a noble death.

Personality
Tobirama took great pride in his abilities. He also had a sense of humour, even under stressful situations. Like his brother, Tobirama felt a great deal of loyalty to the village, and placed its well-being above his own. Tobirama was also described as being strong-spirited, and passionate.

Appearance
Tobirama was a tall, fair-skinned man with white, shaggy hair, red eyes and three red markings on his face — two under his eyes and one on his chin. In most cases, his attire took the form of the standard shinobi dress of his era, consisting of blue traditional armour — similar to that of samurai — albeit featuring a distinctive white fur collar, worn over a simple black suit. This armour was constructed from numerous metal plates, formed into multiple protective guards along his body, in particular: chest, waist, shoulders and upper arms. This clothing was accompanied by sandals and a happuri engraved with the Konoha emblem, in the place of the more traditional forehead protector.

Outside of this outfit, Tobirama wore a simple, short-sleeved, blue shirt kimono with mesh armour underneath and held closed by a pale yellow sash, as well as blue pants and a bracelet of sorts on his left hand.

Abilities
As the Second Hokage, Tobirama was undoubtedly an immensely powerful shinobi and was hailed as one of the most powerful shinobi of his era. During his lifetime, Tobirama created the Summoning: Impure World Reincarnation technique; however, some time after its creation he labelled it as a kinjutsu and never mastered it himself, though he did use it enough for it to be recognised for it by others. By simply touching the ground with his index finger, he could detect the presence and number of enemy shinobi, and according to Minato Namikaze, Tobirama was highly skilled in Space–Time Ninjutsu. Tobirama was also shown to have great physical prowess and a considerable amount of skill in the area of taijutsu, as well as good teamwork with his elder brother, Hashirama Senju. In the anime, it was shown that he was also a powerful genjutsu user, able to shroud the area in darkness that even the Third Hokage couldn't dispel. It was said that he wielded the Sword of the Thunder God, a blade with electrical abilities, and was said to have mastered its use despite the blade's lethal nature.

Nature Transformation
Tobirama was primarily known for his mastery of Water Release. He could perform powerful water-style techniques in areas that lacked bodies of water by creating his own water out of thin air or producing it from his mouth, and perform some of the most complex water-style techniques, such as the Water Release: Water Dragon Bullet Technique, with a single hand seal. This amongst other reasons is why he is revered as the greatest known Water Release manipulator in the series, only surpassed by Mei Terumī, the Fifth Mizukage, who can do that technique without needing hand seals.

Invasion of Konoha Arc
Both he and Hashirama were reincarnated by Orochimaru, using Impure World Reincarnation, to fight Hiruzen during the invasion of Konoha. While fully aware of what was going on, and saddened that he would have to fight his former student, Orochimaru employed a special seal into the back of their heads to rob Tobirama and his brother of their free will. Hiruzen quickly resolved to kill his masters and used a very powerful fire technique that was easily countered by one of Tobirama's walls of water that was then turned into an attack that was stopped by Hiruzen's own earth wall.

During the battle, he demonstrated great teamwork and combination attacks with his brother, forcing Hiruzen to summon Enma for assistance. Hiruzen was however in his old age unable to defeat the two Hokage, as any damage that was done was simply regenerated and was further inhibited by his desire to not hurt either. His soul, along with his brother's, was eventually sealed by Hiruzen when he combined the Dead Demon Consuming Seal and the Shadow Clone Technique to strike both of them at the same time. Before his soul was completely removed, he and his brother apologised to their former student for the trouble they had caused.

Legacy
Tobirama's legacy in the ninja world at large, is mixed. He was known for building Konoha's infrastructure and revolutionising the ninja system on a whole by establishing organisations such as the Academy, ANBU, as well as the Konoha Military Police Force — a judicial system for the village which he had the Uchiha govern, as a sign of trust. He also believed greatly in the Will of Fire of the village, which he passed on to the next generation.

Somewhat conversely, he was also responsible for the creation of the Summoning: Impure World Reincarnation technique, which has been used by the traitors of Konoha: Orochimaru who used the technique to reincarnate both him and his brother to fight Hiruzen in an attempt to destroy the village, and Kabuto Yakushi who used it to aid Tobi in the war against the Five Great Shinobi Countries. Despite this, Tobirama is still revered as a powerful shinobi and Hokage, but still cursed for his creation of the Impure World Reincarnation technique.

Trivia

 * means 'the space between two doors'. are seen as the access to progress and improvement which alludes to his role as the Hokage that built up Konoha's infrastructure. '扉' is also used to refer to title pages, tying in with the Hokage title, as a Hokage is as representative for the village as the title page is for a book.
 * Tobirama is a playable character in the Naruto: Ultimate Ninja series and in Shinobi Retsuden III.
 * Tobirama, along with Hashirama, Minato, and the Konoha symbol, had his face appear in the ending omake of Shippūden episode 129 within the fireworks.
 * According to the databook(s):
 * His favourite words were.

Quotes

 * (To Hiruzen) "Listen Saru. Guide the people, and believe in them, for it's from among them that one will come who will carry on when your time is done."
 * (To Hiruzen) "Saru, you must protect those who have faith in you and who love the village, and train up to those to whom you can entrust the next generation… from tomorrow, you will be the Hokage…!"