Tobirama Senju was the Second Hokage of the Hidden Leaf Village and a member of the Senju clan along with his brother, Hashirama, the First Hokage .
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 the Hidden Leaf Village. When Hashirama 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 Ninja Academy, the ANBU, the Chunin Exams and the Leaf's 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 an alliance between the Hidden Leaf and Hidden Cloud villages. However, Kinkaku and Ginkaku, the Cloud's Gold and Silver Brothers, attempted a coup d’état against the Second Raikage and were defeated by Tobirama, who was left near death after 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 Danzo 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.
As the Second Hokage, Tobirama was hailed as one of the most powerful shinobi of his era and was also the creator of the Summoning: Reanimation technique; however, some time after its creation he labelled it as a forbidden jutsu and never mastered it himself, though he did use it enough for it to be recognised by other Kage. By simply touching the ground with his index finger, he could detect the presence and number of enemy shinobi. Tobirama was primarily known for his mastery of Water Style. 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 Jutsu, with a single hand seal. This, among other reasons, is why he is revered as the greatest known Water Style manipulator in the series.