Bleach features a massive cast of characters, and many of the deepest and most compelling are black-robed Soul Reapers. As Rukia Kuchiki demonstrated early on, these people are monster hunters and exorcists, purifying Hollows and escorting Pluses to the Soul Society. But Soul Reapers have their warm side too -- even Captain Yamamoto himself.
Captain-General Genryusai Shigekuni Yamamoto is the man who founded the entire Gotei 13 and formalized the very concept of Soul Reapers being tasked with maintaining law and order in all the realms. He is many centuries old and is intimidating to behold, but he can be compassionate, and his side of the story is more sympathetic than fans might realize.
Captain-General Yamamoto is primarily bound by his duty, and he takes it very seriously. But just like Captain Byakuya Kuchiki of Squad 6, there is more to Yamamoto than sheer tradition and discipline. Both characters are not only acting out of duty but also making amends for past mistakes, eager to atone and prove themselves honorable. Byakuya broke Soul Reaper law and married a commoner woman named Hisana, then adopted Rukia into the family later on. During the Soul Society arc, there were no real insights into Yamamoto's motivation aside from maintaining law and order, but his backstory was finally revealed later on.
The Thousand-Year Blood War arc shed some light on Yamamoto's role in Soul Society history, and his current adherence to discipline and law contrasted sharply with his personal past. Centuries ago, Yamamoto was powerful but wild, the incarnation of the fury of flames. He threatened to destroy the entire Soul Society until one day, he cooled down (so to speak) and formed the Gotei 13 to help escort souls to the afterlife and maintain order. He'd had enough of his destructive ways and grew determined to distance himself from his past.
Yamamoto even kept a drawing of himself in flames, describing his past self as a horrible monster that once haunted the Soul Society. At least, that's what he said when a young Shunsui Kyoraku inquired about the drawing. No doubt Yamamoto is throwing himself into his work to overcome his guilt about being the worst menace in the Soul Society's history.
Captain-General Yamamoto has his paternal side, though it rarely shows. Late in the Soul Society arc, he confronted Captains Kyoraku and Ukitake, who were among his first-ever students at the Shino Academy. It broke his heart to have to face them in combat. Though he saw them as sons, he was willing to destroy them out of duty, but would never take any pleasure in harming them. Fortunately, the battle was soon called off, and Yamamoto directed his wrath toward Sosuke Aizen and his Arrancar army instead.
The Captain also had similar sentiments toward his Lieutenant, Chojiro Sasakibe. Their relationship was little-known for a long time in Bleach, but early in the Thousand-Year Blood War arc, Sasakibe lost his life at the hands of the Sternritter. In a rare sentimental moment, Kyoraku noted that Yamamoto's grief toward his fallen Lieutenant was deeper and darker than anyone could imagine.
Yamamoto and Chojiro had been allies and friends since before most modern Soul Reapers were born, and a panel in the Bleach manga shows the Captain hanging his head in indescribable grief, his eyes shaded as he silently gazes down at Sasakibe's zanpakuto. It was a brief but powerful scene, and Kyoraku had a similar moment of his own when Yamamoto lost his life against King Yhwach himself. Yamamoto felt guilty about not defeating Yhwach a millennium ago and tried to finish the job again, only to be cut down.
It's up to the survivors and the new Captain-General, Shunsui Kyoraku, to honor the duty and sacrifices of Yamamoto as they fight to protect reality itself from Yhwach's schemes.