Many friendships and alliances drove the Bleach story forward, some of them highly thematic or symbolic. For example, Ichigo and Chad both resolved to be punkish heroes who only use their abilities for the sake of others, helping Chad form a better relationship with his own strength. Then there's the matter of Ryuken Ishida and Isshin Kurosaki.
At first, Ryuken and Isshin had nothing to do with each other. They belonged to feuding factions, but by this point in Bleach lore, the Quincy/Soul Reaper feud is meaningless to the people of Karakura Town. What matters more is being a good neighbor and ally in the fight against evil.
Isshin Shiba, Ryuken Ishida & A Common Cause
In their youth, Ichigo's and Uryu's fathers had no contact with one another, living in totally different worlds. Before Ichigo's birth, Isshin was not even a Kurosaki. He was Captain Isshin Shiba of Squad 10, with Rangiku Matsumoto as his Lieutenant and Toshiro Hitsugaya as his 3rd seat. In the world of the living, the adolescent Ryuken was enjoying a fairly normal life. He had a difficult relationship with his mother but was close to his maid, Kanae Katagiri. One night, Isshin and Ryuken finally crossed paths.
Sosuke Aizen, scheming in the shadows, released an artificial Hollow named White into Karakura Town, which went on a rampage. Isshin was on the case, and Masaki was soon in White's crosshairs as well. She and Isshin both fought White and destroyed it, but Masaki was poisoned with White's Hollow essence, and Ryuken arrived to help. He couldn't save Masaki though, so he brought her to Isshin and a fallen Soul Reaper named Kisuke Urahara, pleading with them for help.
Isshin, with Kisuke's aid, gave up his Soul Reaper powers to save Masaki's life, and Ryuken was grateful. Later, the adult Ryuken married Kanae Katagiri and had a son around the same time Isshin had his own son with Masaki. Ryuken and Isshin were now neighbors of sorts, though they kept their distance starting at this point.
When The Soul Reapers & Quincy Team Up Once Again
During the main events of Bleach, both Ryuken and Isshin -- the latter now a Kurosaki -- were hands-off heroes, allowing their respective sons to save the day instead. The fathers were already symbolic of Soul Reaper-Quincy cooperation, with both understanding that heroism and friendship are far stronger than some ancient feud of a bygone era. Their sons didn't necessarily know of this, but they did echo the alliance of their fathers, with Ichigo and Uryu invading the Soul Society together and later Hueco Mundo as well. Isshin and Ryuken provided only token assistance, but they did have faith in their sons, and that counts for a lot. In particular, Isshin helped take care of Grand Fisher once and for all, slaying his wife's killer at long last. Likewise, Ryuken helped train Uryu to regain his Quincy powers and thus allowed the boy to help Ichigo on his quest to save Orihime.
The fathers shared a few standoffish words when Ichigo and Uryu departed for Hueco Mundo, making it clear that although they were fighting for the same team, they personally didn't get along that well, with Ryuken seeing Isshin as a buffoon and Isshin seeing Ryuken as a lax and coldly distant parent. Still, these two trust one another and are both content to pass the torch to the next generation of heroes. Then the Wandenreich arrived.
Ryuken and Isshin thought their warrior days were done, but not yet. The Wandenreich, led by king Yhwach, was a grave threat, so they both donned their battle gear and arrived on the battlefield during this war's climax. Ryuken even provided Uryu with a special silver arrowhead that could temporarily stop Yhwach's Almighty power and give the heroes a chance.
The plan worked, and Uryu and Ichigo were a perfect team as they took down Yhwach once and for all. Although Ryuken's and Isshin's glory days as warriors were over, they were still in the fight. Their cooperation symbolized the heroic alliance their sons would form, saving the entire Bleach world from a terrible fate. Not bad for two neighbors who tend to annoy each other.
