Chapter 1
Senju Tobirama, although called one of the best shinobi of Land of Fire, considered himself an inventor before anything else. His duty to his clan may have focused his mind towards the shinobi arts, towards the survival of the Senju clan, but at his heart, Senju Tobirama was a creator. It was only a few months after the official creation of Konoha that Tobirama's mind had fully settled into his new mindset: the survival of Konoha. Konoha now included the Senju clan, the Uchiha clan, the Nara clan, the Yamanaka clan, the Akimichi clan, the Hyuuga clan, the Hatake clan, the Inuzuka clan, the Aburame clan, the Sarutobi clan, the Shimura clan, the Kurama clan, the Fuma clan, several minor shinobi families and various skilled civilians. And so it was this final change in mindset that brought up a thought beneficial to the future of Konoha: the Edo Tensei. Originally, Tobirama had developed the jutsu with the thought of reviving and talking to his precious little brothers, Senju Kawarama and Senju Itama. Of course, he wouldn't start with summoning his otouto - what if it caused damage to their souls? So, after contemplation, Tobirama had decided the best place to start would be random civilian villagers. He began his experiments at fifteen and had a workable (but incomplete) version at seventeen. Tobirama had figured out how to ensure the soul was not damaged upon summoning, could be resummoned (so far) a maximum of twenty times, and had a chakra-based regeneration that paired very nicely with the body's ability to convert nature chakra into the summoned's chakra pool, theoretically providing an infinite amount of chakra. Tobirama considered it incomplete for one reason: it required a living person to be sacrificed to summon the target's soul. Now, as a shinobi, it wasn't the morality part that bothered him (Anija had said he had less morals than other shinobi, dabbling in necromancy as he was - which... hurt, at the time). No, what really bothered him was the inefficiency of it all. There had to be a way that didn't require a living sacrifice. Preferably, Tobirama would be able to use the person's dead body to summon them for Edo Tensei. Alas, he was caught by Anija before he could finish the jutsu (before he could summon his otouto, before he could apologise for failing them and letting them die). After many arguments, Clan Head Senju Hashirama (very different from his silly Anija) forbade the use of Edo Tensei for any reason. Now, this didn't prevent the theoretical development of the complete version of Edo Tensei (Clan Heir Senju Tobirama was dutiful and loyal to his Clan Head, he would never disobey a direct order - and he didn't, as he had never used Edo Tensei again), but theoretical results were almost impossible to turn into practical results without experimenting, and experimenting required doing. So, Tobirama had the incomplete Edo Tensei able to be used upon rescinding of his Clan Head's order and several theoretically complete Edo Tensei able to be used upon rescinding of his Clan Head's order. But that was Clan Heir Senju Tobirama - what about Senju Tobirama of Konoha? Now, that Tobirama would require the approval of all thirteen Clan Heads to conduct 'experimentation using living humans for a village beneficial jutsu' under Section 4 of the Konoha Constitution (the other Clan Heads had been baffled at that inclusion, but Anija knew his little brother, yet was practical enough to admit that it might be necessary one day - for the clan for the village).
Theoretically, Tobirama could use a dying person's body to summon anyone, an intact (as in chakra network) corpse to summon the corpse's soul, a living animal to summon anyone (either as the animal or transforming into a human, Tobirama wouldn't know until he experimented with it) or an extensively Fuinjutsu covered puppet to summon anyone (likely with severe limitations to the theoretical infinite chakra aspect of Edo Tensei and therefore the regeneration aspect as well). During Edo Tensei's development, Tobirama had had a stray thought that, at the time, was unrealistic: what if the dead could fight for us? Could teach us? Could forge/farm/create for us? That was unrealistic in the fact that the other clans would band together and wipe out the Senju clan as soon as they understood what Tobirama had created. But now, in the village... was it so unrealistic after all?