Bleach: Thousand-Year Blood War is the anime's final and biggest story arc. This is Bleach's last opportunity to flesh out its main characters' arcs and push them in new directions, such as Byakuya's newfound humility and Ichigo's Quincy ancestry revealed through his late mother, Masaki Kurosaki. Bleach: TYBW also found time to flesh out semi-obscure characters such as Uryu's father, Ryuken Ishida.
Ryuken Ishida first appeared in Bleach as a total kuudere, a responsible but emotionally distant father who disapproved of his son's idealistic ways and Quincy's pride. This strained their relationship, but Ryuken wasn't always so jaded and bitter about the Quincy cause. Ryuken Ishida, as seen in TYBW Episode 11, was once the Quincy tribe's most zealous defender.
Ryuken Ishida's Reasons for Losing Faith in the Quincy Tribe
Despite being a powerful and well-connected Quincy himself, Ryuken Ishida has little interest in the Quincy tribe in the present day. A flashback in the original Bleach anime showed Ryuken and Uryu arguing about the Quincy tribe's place in modern society, with Ryuken rejecting that lifestyle because fighting as a Quincy was simply not profitable. The grounded, pragmatic Ryuken didn't want to fight monsters for the sake of fighting monsters; he had a son to support, so he took a job as the director of Karakura Town's general hospital, or so he told Uryu. There could be more to it, and recent flashbacks reveal how Ryuken lost faith in the Quincy way of life.
Ryuken, who was in his late teens at the time, was confident in his abilities and lifestyle as an Echt, or pureblood, human Quincy. He was extremely protective of the Quincy tribe, discouraging his foster sister Masaki Kurosaki from putting her life in danger in the battle against powerful Hollows like White. Ryuken was determined to keep the Echt bloodline alive, even if it meant marrying Masaki. However, he felt bad about not attempting to marry for love because he truly believed in the power of love and happiness at the time. Ryuken even told his maid, Katagiri, that a person cannot see or strive for the future unless they are happy. As a protective kuudere and a devoted Quincy, he wished to protect Masaki's health and happiness from all threats.
Bleach fans are already aware that Ryuken changed his mind as an adult and rejected the Quincy way of life as meaningless. His practical financial needs as a father are one thing, but it could be something more personal. He had a strong emotional attachment to Masaki, and he must have been devastated when he learned of her death at the hands of Grand Fisher. Perhaps Ryuken was discouraged from training Uryu to fight as a Quincy because he was afraid his son would suffer the same fate as Masaki. Alternatively, perhaps enough of the other Ishidas was killed in battle to persuade Ryuken to cut his losses and keep the remaining Ishidas out of the fray. That could explain Ryuken's feud with his father Soken, who held on to his Quincy ways until the end. Ryuken simply cannot bear the thought of losing anyone else in the battle against Hollows or Soul Reapers, but his son and father insist.
How Ryuken Ishida Might Rekindle His Faith in the Quincy Way
For the time being, Ryuken Ishida remains enraged by the Quincy tribe and the fate of both his family and Quincy as a whole. He is only making a token effort to support his idealistic son Uryu, such as assisting in the explanation of Wandenreich's truth, and nothing more. However, Ryuken's character arc is not yet complete, and Bleach fans have reason to believe that Ryuken will do another 180.
Ryuken finally embraced his Quincy powers again in the "Arrancar" story arc, training his son to give Uryu his powers back on the condition that Uryu never associates with the Soul Reapers again, including Ichigo Kurosaki. Uryu snuck away to help Ichigo with the Hueco Mundo invasion anyway, and Ryuken was unconcerned. Despite his stern words, Ryuken genuinely wished his son well and believed in him, and he didn't mind Uryu fighting alongside Ichigo. He even has an odd friendship with the deredere Isshin Kurosaki, with whom he shares a common bond as the father of a proud shonen protagonist.
The Thousand-Year Blood War story arc is full of unexpected twists and turns, taking its characters in unexpected directions both in the present and in flashbacks. Retsu Unohana was the brutal first Kenpachi, while Byakuya is pleading with Ichigo for assistance, and Ichigo is of Quincy ancestry. It's not a stretch to imagine Ryuken being inspired to fight for the cause and renew his faith in the Quincy tribe, especially if Uryu joins the fight.
Furthermore, if Ryuken despises the militaristic Wandenreich, which he most likely does, this will motivate him to join the fight and clear the Quincy tribe's name. Ryuken was once very proud of his tribe, and perhaps he still is. If that's the case, he'd gladly destroy Yhwach and his thuggish Sternritter to demonstrate to everyone, Soul Reaper and otherwise, what the Quincy tribe is all about: honor, loyalty, and, above all, the loving ties of friendship and family.