The Fiend of Fire and a rather attractive woman with six arms and the body of a snake. She is always depicted this way, although the colors of her body sometimes vary. However, Kary's true self is generally accepted and depicted as the following:
The woman portion of her body is crimson, her eyes yellow with red iris. She wears a blue chest plate, what appears to be two articles of jewelery of some sort around her waist, a blue tiara with a red jewel at the front, earrings and possesses yellow or blonde hair. The snake portion (the lower half) of Kary's body is depicted with purple scales, dark purple ring designs, and a crimson underbelly.
In the first Final Fantasy, she was causing Mt. Gulg - a dormant volcano - to erupt and was the second to be defeated by the main heroes. The player would battle her twice; the first time being in the aforementioned volcano, Mount Gulg. Here Kary would cast Fira and perform powerful physical strikes.
The second time Kary was encountered would be in the Chaos Shrine in the past. About half-way through, stepping on a certain spot would automatically engage in a battle with the fiend. Now stronger than before, she would still attack physically but also cast Firaga, inflicting a great amount of damage upon the entire party.
In Final Fantasy IX, she was the Guardian of the Shrine of Fire, and fought Amarant Coral and Freya Crescent behind the scenes. However, in this game she was called "Maliris" due to a translation error of "Marilith". As the heroes entered Memoria, Marilith was the first of the Chaos Guardians they encountered.
It was in FF IX that Kary (or "Maralith" if you want to refer to her name in this game) not only had quite powerful physical strikes, but could also cast Firaga, Reflect, and, when about to lose, use the Blue Magic "Raining Swords" as a last-ditch effort to defeat the party.
Final Fantasy Tactics Advance had her as a lamia monster working as a Falgabird. However, unlike the other three Fiends (Lich, Kraken and Tiamt), she has not been seen solo in any of the Final Fantasy games. Also, whereas the two final Fiends are named after mythological creatures, a marilith is a demon made up in fantasy fiction. Kary also appeared in Final Fantasy XII as a rank V monster in a hunt quest, though she only appears as a simple snake with her distinct color style.
In 8-bit Theater, a web-comic series by Brian Clevinger, Kary is a fiend who enjoys indiscriminately killing people via combustion, feeling it's the only way to prove she's evil. However, she overdoes it and ends up killing all her minions with little, if any, provocation. Black Mage stated that if Kary were not trying to kill him, and half made of snake, he would be "all over her like blood on his knife."
During her battle with the Light Warriors, she killed Black Belt. After Fighter held the six-armed Kary at bay using his newly-crafted Sword-Chucks, Red Mage managed to stuff her into a bag of holding and cast Ice-9, an enormously powerful Ice spell capable of freezing everything in the entire world, into it.
White Mage then shattered the frozen bag with her hammer, destroying the bag and all its contents, including Thief's collection of "more riches than actually exist." Kary was last seen together with Lich, greeting their fellow fiend, Kraken, as he entered Hell.
1. Kary: Minions! What is taking you so long? Do you wish to experience my ire? Let me assure you it's no conicidence it rhymes with FIRE!
2. Kary: How can this be! I have a host, a legion of foul beasts summoned from the ashes of a thousand evil fires burning from a thousand evil trees stolen from the evil forest of Bunny Town.
3. Black Mage: DAMMIT, Thief!
Kary: What are you doing?
Red Mage: We're escaping?
Kary: Is that a fact?
(Kary raises her arms into the air, saying "Wall of Flames". A colossal wall of fire appears and forms a large ring, surrounding the area.)
Red Mage: Aww.
4. Kary: Look, I haven't killed anything in about three minutes, so my patience is NIL at this point.
Black Mage: She seems reasonable. Let's reason with her using our reason.
5. (Red Mage, White Mage and Black Mage are standing over the shredded corpse of Black Belt, looking at it.)
Kary: At least his death wasn't in vain. He stopped me from venturing out of my volcano to murder every living thing I find. I mean, obviously I'm going to go do that NOW, but he stopped me for nearly a whole minute.
Red Mage: Red Mage: Well, you've sure beaten us, Kary. Nothing left to do but - ICE!
(Kary is immediately encased in a block of ice. However, it then shatters, an angered look on Kary's face.)
Kary: I'm living fire. You didn't think it was going to be that easy, did you?
Red Mage: (looking down.) You know, for a second there, yeah. I kinda did.
6. Red Mage: Okay, YOU stall Kary while I think.
Kary: Good luck with that. I can make your organs explode with but a thought.
6. Ur: What the Hell is this?
Kary: You answered your own question, pal.