Warning: There WILL be unmarked spoilers. If you do not wish to be spoiled, please do not read this.
Shigekuni Yamamoto Genryusai is the general of the Gotei 13 and the captain of the 1st Division. Despite being old, he is one of the strongest shinigami in Soul Society, and is highly-respected among all shinigami.
By merely staring with spiritual pressure, he was able to immobilize the 8th Division Vice-Captain, Nanao Ise. He was also able to take on two captains simultaneously, Kyoraku Shunsui and Ukitake Jushiro.
The shinigami academy was founded by Yamamoto himself, about 2,000 years before the main Bleach storyline. He was also a teacher back then, who cultivated his students into great shinigamis such as Shunsui and Uktitake.
Shigekuni Yamamoto Genryusai’s Zanpakuto
Named the Ryujin Jakka, it is the oldest and the most powerful fire-based Zanpakuto ever known. It is an attack-oriented Zanpakuto, probably making it the Zanpakuto with the greatest attack power in all of Soul Society.
In its sealed form, Ryujin Jakka appears to be a harmless walking staff. When released, it turns into a katana with a dark red handle and a circular handguard.
Upon the command of “Bansho issai kaijin to nase” (All things of this world, turn to ashes), Ryujin Jakka goes into Shikai (Initial Release). Yamamoto himself and the surrounding area immediately gets engulfs in strong flames. With a wave of the Zanpakuto, intense fire burns whatever is in its way.
Yamamoto’s Bankai (Final Release) is unknown at this time.