Genryūsai
Shigekuni
Yamamoto

Genryūsai Shigekuni Yamamoto founded the Gotei 13 and commanded it for over two thousand years. He built the Shin'o Academy, trained Shunsui Kyōraku and Jūshirō Ukitake personally, and set every rule the organisation still runs on, including the ones he later has to enforce against people he raised.

Ryūjin Jakka is the oldest and strongest fire Zanpakutō in existence, and the reason he almost never has to use it. Releasing it in the Seireitei would burn the Seireitei, so most of his appearances consist of standing still while other people decide not to fight him. When he does commit, in the fake Karakura Town, he seals himself and the flames inside a wall of fire rather than let it spread, and Wonderweiss Margela was built by Aizen for the single purpose of swallowing it.

The founding version of him was not the man in the meetings. Before the Gotei 13 there was a squad that killed for the sake of it, and Yamamoto led it. What he becomes afterwards is a deliberate construction, which is why he is so unyielding about procedure. Yhwach kills him in the first invasion, having sent Royd Lloyd wearing his own face to learn how Ryūjin Jakka worked before ever facing it.