Except for the last one, these are all Haikus.

Namine'

"Witch", DiZ called her

Namine', Nobody

-of, oddly, Kairi

Riku Replica

Riku Replica

Created by Zexion

Hated all the lies

DiZ

Darkness in Zero

Formerly Ansem the Wise

Source of the Heartless

Xemnas

The Superior

Nobody of Xehanhort

"Ansem's" Counterpart

Xigbar

Controller of Space

Aptly called The Freeshooter

Often flys, guns blazing

Xaldin

Element of Wind

The one Whirlwind Lancer

Six lances in battle

Vexen

An Ice Controller

The Chilly Academic

His weapon? Shield.

Lexaeus

A Stone Powerhouse

The Silent Hero

A giant tomahawk

Zexion

An unknown element

The enigma, Cloaked Schemer

An unknown weapon

Saïx

Moon elemental

Berserk Luna Diviner

Slams with his claymore

Axel

He controls fire

The Flurry of Dancing Flames

Chakrams by his side

Demyx

A Hydromancer

The Melodious Nocture

A sword guitar fusion

Luxord

A Time controller

Takes risks; Gambler of Fate

Magic, his ally

Marluxia

Florakinetic

A true Graceful Assassin

With a sharp, pink scythe

Larxene

An electric sadist

Daggers are her true weapon

The Savage Nymph

Roxas

His element's Light

He's the Key of Destiny

The last of the group

Organization XIII

A mysterious force

That let Darkness run its course

They wished to revive him

The powerful Ansem

Their Superior, his Nobody

Was Xemnas, nowhere near so haughty

They were the body and mind

Of strong people the Heartless did find

They wished to be whole

That was their goal

Through any means

Now they're fiends

Masters of manipulation

They were the Organization.