MyInnerFred isn't entirely right. I'll just go ahead and post all of them as found in Ultimania.
Xemnas: 無 (mu) literal meaning: nothing, naught, nil, zero
Xigbar: 空間 (kuukan) literal meaning: space, room, airspace
Xaldin: 風 (kaze) literal meaning: wind, breeze
Vexen: 氷 (hi) literal meaning: ice, hail
Lexaeus: (tsuchi) literal meaning: earth, soil
Zexion: none given
Saix: 月 (tsuki) literal meaning: moon, month
Axel: 炎 (honoo) literal meaning: flame, blaze
Demyx: 水 (mizu) literal meaning: water
Luxord: 時 (toki) literal meaning: time, hour
Marluxia: 花 (hana) literal meaning: flower, petal
Larxene: 雷 (ikazuchi) literal meaning: thunder
Roxas: 光 (hikari) literal meaning: light
Those are all the elements taken from KH2 Ultimania. Though the final determinants will be what they have listed in the English game. And since I'm assuming they just translated everything from the game, they'll most likely list the elements found in the game (Darkness, Time, Shadow, Illusion) as opposed to the ones found in Ultimania (Nothingness, Space, Time).