Disclaimer: Bleach is the property of Kubo Tite. Written for fun, not profit.
Sticking around the Soul Society for any length of time requires a firm and distinctive sense of self - which might explain why the captains of the Gotei 13 wobble between eccentric and batshit insane, or it would explain it if Isshin gave the matter a moment's thought. But all he knows is that he is (was) one of the older shinigami, that he's somewhere between four and five hundred years old, and that all but the first few days of this have been spent dead.
Masaki calls him a cradle-robber (in the nicest sweetest way possible, of course). When he tries to round his age down to three-fifty, she amends that to a cradle-robber and a bad liar.
Kurosaki is the name of an utterly blameless man who lives on the same street as Isshin in the 48th district when he was a child - a very nice fellow, if a bit bland, and if he's still around he probably doesn't deserve the questions he gets when Isshin's darling boy finishes upending the afterlife.
The point is that not so long ago Isshin had a different name, which is more well-known than Kurosaki but considerably less unremarkable and definitely less blameless, and most days he remembers to forget it ever existed.
The best part about being a captain is wandering into the living world to escape the paperwork and telling random underlings that he was making surprise inspections. The expressions on their faces are priceless.
Isshin doesn't have many regrets, and most of the ones he does have revolve around Masaki and their children - that he isn't a better husband and isn't a better father, that he wasn't stronger, that he never thought about the danger he might put his loved ones in simply by existing.
After them (and after his family everything else is a distant, unimportant second) he regrets what happened to the Quincy. He even tries to befriend Ryuuken to make up for it, at least until he realizes Ryuuken is an asshole.
When he starts at the academy he had three goals in life:
a. Drink the rest of the class under the table.
b. Get in Yoruichi-taichou's pants.
c. Be universally adored and worshiped, preferably by beautiful women.
He graduates 0.5-for-3. Convincing most of his (intoxicated) classmates to (drunkenly) serenade the captain of the First Division in a fairly obscene manner has to count for something.
His very first division is the Fourth. To this day Unohana is the only being in the universe who can reduce him to a state of gibbering terror.
Gin annoys the shit out of him and Aizen always strikes him as the type to sort paper clips by size - which is to say Isshin ignores him completely - but every now and then something about Tousen's choice of words creeps him the fuck out. Later on he likes to think this means he got one out of three right, but he knows he's a bad judge of people.
(He's fairly certain he's reading Urahara correctly, but he wouldn't be surprised if he winds up drawing a weapon on the scheming bastard, either. Especially if he keeps dragging Ichigo into things.)
Masaki can't see him, not the way Ichigo can see ghosts just like people. She tells him she sees afterimages and flashes of movement at the edges of her visions - and that she can hear him just fine thank you very much, there's no need to shout.
She agrees to marry him before she knows what he looks like; right before the wedding she smiles like sunshine and tells him she thought he'd be taller, and he knew this is worth everything he gave up ten times over.
His captaincy isn't the shortest on record, but it's close.
The highest compliment he could ever pay his children is saying their mother would be proud of them.
(He is too, but he knows that doesn't count.)