Kingdom Hearts: CoM » The Organization

Vexen (No.4)
Element: Ice
Weapon: Shield
He was the researcher of the heart and the creator of Riku Replica. At Twilight Town in CoM he told Sora about the other side of his heart. He was then killed by Marluxia.
Written by Itani.
Lexaeus (No.5)
Element: Earth
Weapon: Tomahawk
Lexaeus was well built and wielded the element earth. He had the same intentions as Marluxia but was using Riku instead. He was killed in battle by Riku.
Written by Itani.
Zexion (No. 6)
Element: ??? (Poison?)
Weapon: None
Zexion was a guy that didn't want to take part in combat. After the deaths of Vexen and Lexaues. He was forced to fight Riku and disguised himself as Sora. But by then. Riku master the power Darkness and saw through the disguise.
Written by Itani
Axel (No. 8)
Element: Fire
Weapon: Charkams
He is a shadow whose intentions and secrets are hidden from all. Axel has no qualms about exploiting others to suit his needs. He used Sora and Namine to silence Marluxia and halt his plot against the Organization. Axel claims that he and Sora have something in common, but that connection is unclear.
Marluxia (No. 11)
Element: Wind
Weapon: Scythe
He wanted to use Sora as a puppet, so he could take over the Organization even though he was a low ranked member. He ordered Namine to erase Sora's memories of Kairi and replace them with Namine.
Written by Itani.
Larxene (No. 12)
Element: Thunder
Weapon: Kunai 2
Killed by Sora in battle. Larxene wielded the element of Thunder and her choice of weapon was two Kunai. Same story as Marluxia, but Larxene also tried to get Axel to join them.
Written by Itani.
Element: ???
Weapon: None
Truly an enigma. DIZ appeared before Riku in the guise of Ansem, presenting him with choices regarding the darkness within his heart. It is said that he and the king have met.

Xarvenu's Corrections:
The correct name for Axel's weapon is Charkams.

Marluxia's element is flowers according to Nomura and his Weapon is spellt "Scythe".

The correct name for Larxene's daggers are Kunai.