"As lightless oblivion devours you, drown in the ever-blooming darkness!"
Little is known about Marluxia's background before he became a Nobody. When first "discovered" by older members of the Organization, Xaldin and Xigbar, Marluxia is made to be the eleventh member. Some time later, Larxene becomes the twelfth member, and Marluxia "befriends" her. Except not really, so it's more like "manages to get along with", to the point where they get to agreeing that the castle is boring, that Nobodies like them are better off without hearts, and they should totally take over the Organization together because the older/original members suck. Yes, talking to Larxene did all of this. Blame her. Then, soon enough, Roxas joins the Organization. With plans already forming, Marluxia is all too happy to help out Roxas understand his purpose of destroying heartless and collect hearts. Somewhere along the line, Saix becomes aware of plans to overthrow the Organization, and sends "suspicious" members to Castle Oblivion, along with Axel, who is given the orders to kill anyone who has these plans. Unaware of Axel's orders as a double agent, Marluxia and Larxene continue with their schemes. Like Team Rocket.
For some reason, Marluxia is put in charge of Castle Oblivion. Most likely because Saix had figured it would draw out the other's true intentions, and yes, it in fact does. First luring Sora to Castle Oblivion by appearing before him one night, and then telling him what he seeks is within the Castle, Sora is successfully caught. Then, using Namine, a Nobody who can manipulate memories, Marluxia coerces her into planting fake memories in Sora's heart, in order to confuse him into thinking she was someone he had to protect. Then by extension, he would be able to control Sora's actions by threatening Namine, and hope to use Sora to defeat the original members of the Organization without needing to do so himself.
Things seem to be going according to plan for a while, Sora slowly begins to forget, and instead gain "new" false memories. That is, until Vexen comes in and starts to screw things up, by making a replica of Riku in order to try and gain the upper hand on Marluxia. The replica runs away from Vexen, and Marluxia threatens to tell "The Superior" about how much fail Vexen is made out of. Vexen in return goes to fight Sora to prove his worth, and this is where things start to screw up. Vexen instead shows Sora a memory that was supposed to be repressed, and upon seeing this, Marluxia gives Axel the order to "rid us of our traitor." Axel, knowing at this point that Marluxia and Larxene are also traitors to the Organization, grins. He ends Vexen's life, and Marluxia and Larxene decide to allow the redhead into their schemes.
And that's when Axel starts to mess up everything ever. Marluxia and Larxene go to celebrate Vexen's death, and leave Axel to watch over Namine. Axel then proceeds to convince Namine to put an end to the madness, and he doesn't mean for her to kill herself. Larxene sees at this point that Axel's a backstabbing bastard, and goes to fight Sora. She promptly loses and dies. Marluxia and Axel begin to fight elsewhere, and appear to be equally matched, exchanging words while they do so, until Marluxia finally summons Namine to attempt to use her as a sheild. Enter Sora, who comes in just as Axel's saying how he doesn't care if she gets hurt, and Marluxia tells Sora to get rid of Axel, if he wants to save Namine. He then disappears, off into another room with Namine, demanding she erases Sora's memories entirely. She refuses, he gets pissed, and fights against Sora. But, turns out, it was just a clone. Sora goes through the doors in front of him to engage Marluxia in one (or two, depending on which version of Chain of Memories) more fight, until he is finally killed once and for all.
Manipulative to a fault, Marluxia won't hesitate to use anyone as a tool for furthering his goals, but this isn't atypical behavior for a member of the Organization. Without emotions to cloud thought, any action taken should be to further one's own goals, and Marluxia has a ferocious dedication to his own goals. Manipulations wise, he influences Namine's feelings of loneliness, convincing her that Sora is her knight in shining armor. He then uses Sora's loyalty to Namine to push the Keyblade bearer into attacking Axel. But manipulating small children isn't all he does, he also coerces Vexen's pride by telling him to prove his worth to the Organization by attacking Sora.
While his gestures and choice of words suggest that he likes to be overly dramatic, he doesn't pretend to have emotions often (much like Saix and Xemnas), in either the game, or the manga. Also like Saix and Xemnas, the few times he does display emotion are either in laughs, smirks, (or in the manga's case, total rage). This is probably due to him realizing he'd rather not have a heart, and by not wanting one, he doesn't bother to pretend he does.
The drive behind Marluxia's struggle to control the organization lies within his obsession with his own superiority. As number eleven out of THIRTEEN, he's feeling a little bummed out that because he arrived so late, he is automatically treated with less respect. This, paired with Larxene apparently feeling the same way, gives him the opportunity to rebel against the founders of the Organization, in order to prove himself more powerful than the "Superior" may understand. Vexen's "pulling rank" rant only served to trigger Marluxia's superiority complex, and nearly stabbing Vexen with his scythe right then and there.
And last but not least, Marluxia is an arrogant son of a bitch. He knows he's strong, he knows he's pretty, and he will blatantly show off both just for the sake of showing off. Some organization members take off their hoods, Marluxia brushes his hood off of his head, flipping his hair, and spreading petals all over the place while doing so. The roses he'll show up holding for no reason? It's entirely unnecessary, but he does it anyway.
As every Organization member has the power over some element, Marluxia's rests with plants, but more specifically, flowers. In fact, he tends to walk around with just a cloud of petals floating around him, for absolutely no reason other than he probably thinks it makes him look pretty.
His signature weapon is a large green scythe with a rose pink blade. It's named Graceful Delila and isn't that just the girliest thing ever. But regardless, it has a lot of attacks besides just slashing at people. It can send out an energy wave (Crescent Wave), and a shockwave like attack (Circle Reject), as well as a few attacks that rush at the victim from above, one specifically called Blossom Shower, that releases another shockwave, and a ton of attack petals. Basically, Marluxia's twirling around with a scythe and stabbing you with razor sharp flower petals, in short, you're screwed.
Other abilities that worked in Castle Oblivion but probably won't work in Nautilus are: creating a clone made of petals, levitating, teleporting around like a mofo (without using a darkness corridor) when you fight him, and summoning a giant mech that looks like a weird kind of weed whacker/praying mantis combination, if the blades were really scythes and pink, and Marluxia was it's head. Also summoning a giant Grim Reaper like figure which has the abilities like "Omni Laser", "Doom" and "Whirlwind to the Void".
Important relationships and connections.