Okay, so this is going to be the first part of me looking way too deep into the character and themes of Marluxia, and I decided to start off on some things relating to his Attribute.
I am going to very bluntly come out and say what is probably obvious. Marluxia's attribute being 'Flowers' is symbolic.
His true attribute is 'Death'.
He wields a scythe and the high-ranking Nobodies under his command are called 'Reapers'. Flowers, due to their seasonal nature, are quite commonly used as a symbol of the beauty and fleeting nature of life. In Japan the traditional flowers for the symbolism are cherry blossoms, and the flower petals that accompanied Marluxia originally were those of cherry blossoms until they were changed to be rose petals, which is another flower commonly used for the same symbolism (although roses themselves have a plethora of other symbolisms as well).
All in all, the themes provided by his mere presentation alone are quite on the nose, but then we get to the very specific trademark ability he has.
Doom.
Marluxia has one of the more infamous abilities from the Final Fantasy franchise, Doom. Or, as it is called in Japanese, Death Sentence. A counter is put on the target, once it hits zero the target instantly dies regardless of how much HP they have. The death connection is rather self explanatory.
The visuals that accompany his casting of this ability, though I'm focusing on his Re:Mind data fight here, keep emphasizing it further. When he is about to apply Doom, he floats up into the air while his scythe swings behind him like a pendulum (a visual representation that feels like a "Your time is running out" -message) and he has the line: "The end is nigh" in EN, and when translated, his JP one is: "This is the beginning of the end". After which, he applies the Doom counter and proceeds to cover the entire arena in one of his vortex puddles. And, after a few seconds, a skull, another symbol heavily associated with death, rises out of it. As an added side-note, the angelic avatar behind him during his third phase in Re:Com also wears a skull mask, although it's not pulled down to cover its face.
I also want to mention the way he applies the Doom counter in both of his data fights. He appears behind Sora and whispers into his ear. This might be a reference to Roman triumphs (I'll elaborate on why I came to this conclusion when I talk about his names) where a person had the job of whispering into the ear of whose triumph it was while standing behind them. The whispered phrase was "Memento Mori".
While it is impossible to tell what he is saying in KHIIFM fight due to the lip movement only being them opening and closing, the tempo of it could fit him saying the phrase. This doesn't apply in his Re:Mind data fight though, as we see a more detailed lip movement from him, but what the line could be instead, assuming it's in English, is "Remember how mortal you are", which would still work, as the Latin phrase means "Remember that you have to die".
I also want to briefly touch upon his scythe, The Graceful Dahlia, as it appears to be a rather peculiar weapon even among the Organization, especially when we look at his data fight in KHIIFM. Namely, when Marluxia lands a hit with Dahlia, it subtracts from the counter, while every other of his attacks drain health. This could potentially mean that Dahlia doesn't exclusively (although it can do so considering how hard the damn thing hits in KHIII) damage the physical body, but could damage the target's spiritual body (the soul) as well. This would further tie into Marluxia's 'Grim Reaper' theme as it could refer to the act of reaping souls, and it would arguably make his scythe one of the most dangerous and volatile ones of the Organization's weapons. But I won't get further into the topic on his weapons here as I'll analyze those separately.
And we have now reached the end of this post. This is meant to lay some of the groundwork for the perspective I'm approaching Marluxia's character with, and what I keep in mind as I analyze the themes around him.
