Brutality

Brutality, by Gundam Kaizer.

This takes place in Kingdom Hearts 358/2 Days, with the premise that because Axel was sent on a mission the day before Xemnas found Roxas, Larxene was chosen as Roxas's mentor instead.

From there, Roxas begins to take on Larxene's aggressive nature, and things in the Kingdom Hearts verse start changing a bit more and more with each chapter....

"(Larxene would) ... get him used to holding a weapon... one way, or another."
 * Alternate Universe Fic
 * Anti Hero: Roxas.
 * Axe Crazy: As of chapter 2, Roxas is showing signs of either this or Blood Knight.
 * Badass: Pretty much everyone in the Organization is Badass to one degree or another, but in this universe, it seems to be played up a bit. The prose even uses the phrase "as though Roxas were already a shadow of the badass he would eventually become."
 * Calling the Old Man Out / What the Hell Hero: In the second chapter, after Xion's introduction, Xemnas asks Larxene for a report on Roxas's progress, and on whether or not he's ready for missions yet. They have a brief conversation about Roxas while completely ignoring/forgetting his presence. Roxas, getting frustrated, interjects, "I'm sitting right here, you know. Don't talk about me like I'm not present!"
 * Comes Great Responsibility: Played with. Roxas, before an off-screen training session, remembers that Larxene once told him that "With great boredom, comes great cruelty potential.""
 * Darker and Edgier: Not enough to warrant more than a T rating (so far), but Roxas's debut fight against some Heartless Mooks in the second chapter is notably more savage than most fights in Kingdom Hearts canon.
 * Deadpan Snarker: Various members of Organization XIII, including Marluxia and Zexion.
 * For Want of a Nail: Axel was sent on a mission just before the Organization found Roxas, forcing Xemnas to find a replacement mentor.
 * Hot Blooded: Despite being a Nobody, Roxas is notably much more emotional than he was in canon, especially with regards to frustration. See Axe Crazy and What the Hell Hero for details. Given that he's a Nobody, and his "emotions" are hinted to be genuine rather than an act, Gundam Kaizer has some explaining to do.
 * Irony: In the first chapter, Xemnas decides that, for a change of pace, he'll let his peons vote on who gets saddled with training/rasing Roxas, and himself abstains from voting. It ends up a tie between, and Xemnas ends up having to be the tiebreaker anyway, and he decides that the entire thing was a waste of time.
 * Jerkass: Larxene, in spades.
 * No Social Skills: Roxas, at least for now. Justified, since he's only had about a week to try and develop any.
 * Pet the Dog: In a roundabout way. Larxene, making a report to Xemnas in the second chapter, says that he "could probably fetch the laundry by himself..." Which, to be honest, IS a step-up from being totally worthless not long before.
 * Precocious Crush: Roxas appears to have one for Larxene, despite her insulting and degrading him constantly.
 * Reasonable Authority Figure: In chapter 1, Gundam Kaizer seems to paint Xemnas, Xigbar, and especially Zexion as these.
 * Rebellious Spirit: Minor example. From what we've seen so far, Roxas is ALREADY rebellious enough to yell at his boss, but not enough to cause any real trouble.
 * Took a Level In Badass: Roxas, at least compared to canon at the same timeframe. Apparently Justified in that while Axel didn't do anything with Roxas's first week in canon, Larxene's implied to have been forcing him to defend himself against her attacks.


 * Training From Hell: Offscreen, but heavily implied to be the reason for the above trope.
 * Tsundere: Larxene appears to be something of a tsuntsun type in the vein of Asuka, although it's (so far) mostly just degrading Roxas.
 * Video Game Cruelty Potential: Invoked by Larxene, if you'll excuse that none of the characters are aware that they're actually IN a video game.