Second Hokage

Tobirama Senju was more passionate and enthusiastic compared to his calm and prudent older brother, the 1st Hokage. And he was the Jonin master of Hiruzen, Homura, and Koharu.

After the leader of the Uchiha clan tried to usurp control of the village, Tobirama became distrustful of the Uchiha, worried they may eventually follow Madara's footsteps, so he appointed them the village's police force as a show of good grace, however he had ulterior motives, getting them to gather in one place in order to better keep an eye on them. Some Uchiha suspected that Madara may had been right, that they would be subjugated by the Senju.

While on a mission with his team along with Danzo and his teammates Kagami and Torifu, they were being tracked and were greatly outnumbered by the Cloud village's elite Kinkaku Force. It was suggested that one of them would have to serve as a decoy for the others to escape and Hiruzen volunteered, however Tobirama told them that he would be the decoy. Danzo insisted against it, saying that he's the Hokage, however he passed the mantle of Hokage to Hiruzen and gave some words of wisdom before he left. He would ultimately be killed by the Two Shining Lights.
 

Many years later, Orochimaru discovered how to use Tobirama's Resurrection to the Impure World and brought back both Tobirama and Hashirama, which impressed Tobirama. The reunion however was short-lived when Orochimaru forced them to battle Hiruzen. At the end, Hiruzen used Minato's Reaper Death Seal to kill him and Hashirama, trapping them in the Shinigami.

Powers/Techniques

Tobirama was a master water user, even able to use water Jutsus without needing a source of water. The full range of his abilities are unknown, but he has demonstrated a few as Orochimaru's undead servant, such as: "Water Encampment Wall" where he creates a protective wall of water around his body. "Water Collision Destruction" where he creates a large tornado of water. "Water Jet" where he creates a thin stream of water that can cut through rock. "Liquid Chains" where he'd bind his opponent's limbs and drags them underwater. "Underwater Motion" that allowed him his travel quickly under water. And "Water Dragon Missile" where he creates a powerful stream of water shaped like a dragon; but while it normally requires many hand seals to use, Tobirama was able to use it with only two. 
Minato also implied that Tobirama could teleport.
But perhaps Tobirama's most fearsome jutsu that he created was "Resurrection to the Impure World," where he could sacrifice the living in order to resurrect the dead as his servants.

Tobirama also possessed the "Raijin Sword," a sword that could generate electricity.