Authors note: This is probably all wrong, but it is so fun to speculate.
Don't own anything.
Big Brothers
Isshin was not looking forward to the day when one of his little girls brought home a boy. When that day came, however, Isshin knew he would have an ally because big brother Ichigo would never allow ANYONE to hurt his sisters in ANY WAY. And with his reputation as a troublemaker and his looming muscled physique, Ichigo made a very threatening figure to any potential ill-intentioned suitors. When the right guys do come around, Isshin knew Ichigo would become their big brother, too, because Ichigo liked being everyone's big brother, and because Ichigo reminded him of another big brother he knew.
When Masaki first told him that she wanted him to meet her brother, Isshin had been very confused. Masaki was an orphan with no close surviving relatives. It wasn't until he saw them together, that Isshin understood. Masaki and Ryuken may not have been blood sister-and-brother, but the two Quincy had become that all the same.
Isshin's first impression of Ryuken… sullen, snarky, over-protective bastard.
The first meeting went horribly. After Ryuken's umpteenth insult to his general appearance, Isshin ended up tackling Ryuken and getting the food all over Masaki. The second meeting didn't go much better, but at least it was Ryuken that got in trouble for bringing out his bow when he mistakenly though Masaki wasn't watching.
In the end, Isshin could tell that Ryuken cared, and anyone who cared for Masaki as much as Ryuken did was a good guy in Isshin's books (annoying as hell to deal with, but a good guy). It took a bit of time, but Isshin eventually convinced Ryuuken that he cared about Masaki too, even if he was a shinigami. Isshin understood that there were some things about being a Quincy and Masaki's time with the Vandenreich where he would never be able to comfort and understand Masaki the way that Ryuken was. Ryuken came to understand that Isshin was not nearly as much of a flake as his goofy sense of humor of led you to believe. Somewhere along the lines, Ryuuken became one of Isshin's best friends.
Ichigo was often a sullen, snarky, over-protective bastard, but it was because he cared.
Isshin could see a lot of himself in his son, every though he looked so much like his mother. Isshin could see a lot of his brother-in-law in Ichigo too.