Well, here's my theory:
Masaki and Isshin are Quincy and Shinigami, respectively.
At the end of the SS arc, Ichigo flashes the substitute badge that Ukitake gave him and Ishida seemed to recognize it as something significant to him (but then thought maybe he was thinking too much into it).
Isshin and Ryuuken are familiar with each other, yet Ryuuken referred to him as Kurosaki (something Isshin said Ryuuken has never called him before) and Ryuuken knew Isshin had shinigami powers. Ryuuken allowed Ishida to help Ichigo, and they also exchanged words on fatherhood, something close friends tend to do.
So maybe Ishida and Ichigo are cousins? Masaki could see the Grand Fisher (or at least his lure) and went to save Ichigo. It's possible her powers were sealed (as were Isshin's, perhaps as a condition for them being together) and she was killed.
It's either that, or Masaki was a VL-level hollow/arrancar a'la Nel that was put into a special gigai by Urahara after some bastard-love-incident occurred between her and Isshin.
Isshin is more than likely with the Royal Guard though, as he always talked of the arrancar and vizard as "we have been watching them" meaning he is definitely high-ranking. I'd speculate he was either a captain in the royal guard, or a high-ranking senior captain OF the royal guard.
But of course,
Isshin will tell Ichigo not to listen to Aizen's nonsense and then Aizen will just-as-planned Isshin, who will attack him, damage him, only to find out he really cut down Ichigo because Aizen always knew Isshin would eventually show up.
Kubo really needs to stop this "Aizen knew it would happen" shit. It gets really boring when the villain doesn't anticipate something and has to think on his feet for a change.
"Unohana. If someone were to show up, I was sure it would be now."
"Ichigo I knew you and your friends would come here, lol"
"I always knew where Urahara's base was"
"Did you really think I didn't know where the Hogo was this whole time?"
"You're just the man I thought you were, Captain Urahara"
"I knew you would come to rescue her, she's useless to me now"
"I knew this wasn't the real FKT already lol"
"We'll beat you without raising a finger"
*100 chapters later*
"I knew the Espada would fail me, now time for me to fight"
"I knew you vizard would show up, it won't make a difference"
"I knew your shikai wasn't as awesome as mine"
"I knew you would go bankai, Captain Hitsugaya"
"I knew you would all attack me so I used KS from the start lol"
"I knew you would wait to assault me, Yamamoto, that's why I made Wonderweiss"
"I knew you would defeat Wonderweiss, that's why I made him go boom"
"I knew you would throw yourself on the attack, now I can kill you"
*whoops, didn't know you would actually fry both of us... WTFICHIGO*
"Lol, I knew you would try to kill me, so I implanted the Hogo in myself."
"Lol, I knew what you were since you were born"
I mean, seriously. That's the entire Bleach manga of Aizen in 30 seconds.