First of all: I do not own Bleach, and I'm making no money from this fic. This was written because Karin, who is pure undiluted awesome, doesn't get enough fandom attention, and because I'm a sucker for cute sibling relationships.
Things Kurosaki Karin learned from her brother, whether he knew this or not:
The correct emphasis, timing, and cadence of a string of cuss words
That you keep your mouth shut about the ghosts thing, cause seriously, like anyone would believe you anyhow
That her mother, who Karin could only remember in the fuzzy snippets of early childhood memory, had kept a jar of candies on the highest shelf in the kitchen – the one they now used for dull old books and knickknacks since even Isshin needed a stool to get at it.
→ → ∆+X, ← ∆+O, and Kurosakis never use Hwoarang 'cause his moves are totally cheap.
"…good. Okay, then what's 6x3? Don't give me that look, dammit. You gotta learn this crap if you don't wanna repeat second grade. Now listen – if you got six people and each one of 'em's got three, uh, apples…"
Don't look over the big fence around the apartment building next to the schoolyard, but she did anyhow. When the transparent girl hanging from the tree turned, rope creaking, to look at her, Karin froze and gripped at her perch on the fence with suddenly numb fingers. It was only afterwards when the recess lady was dragging her inside and telling her off that Karin remembered she didn't believe in ghosts.
That you don't go around picking fights, but if people mess with you, or your friends, or your family, or if they're just really annoying, you kick their asses
That her brother needs to worry about himself more, but there's nothing Karin can do about that besides yell at him when he (inevitably) does something stupid and gets hurt, and go sneak some bandages out of the clinic.
The Kurosaki family kick, which Ichigo had in turn learned from the old man, who claims that it has been passed down by generations upon generations of their ancestors. Isshin has yet to produce any evidence that the Kurosakis were a clan of valiant warriors, however, and Karin is inclined to believe that Isshin is senile anyhow.
That when you say you're going to do something, you do it, and you don't let a damned thing stop you
The look on Ichigo's face when he lies to her about what he's been doing at night and pretends that nothing is going on reminds her of how she feels every time she turns away from a ghost, and says "Oh nothing," when her friends ask what she was looking at. So, really, she has no right to feel so betrayed.